Landlord selling, but won’t sell to me

If you can afford anything like that figure, and you want the house, I'd make an offer. His number isn't a million miles off the zoopla estimate, and you say it's a unique house. I guess it's up to you if you want to open up negotiations or not, because currently you haven't dealt your hand. You have to weigh up against the cost of moving, possibly a couple of times, too. It isn't only the landlord that would benefit from you buying it.
Of course he might still say no, but you haven't actually tried yet.

If you want the house, make a firm & fair offer OP.
(You should ask an estate agent for a valuation yourself imho & offer that or 350K or something in between)

You'll only be able to negotiate over furniture & white goods at this stage. But a fair price paid for something you want, with maybe a few cherries on top is probably better than the prospect of better & continuing to pay rent somewhere . Also the 2 moves will have costs, & be a double PITA.

I'm not sure how you expect a negotiation without making an offer. That would be the logical place to start. He must have told you what the valuation was for a reason...

You’d be mad to offer him the price he says it was valued at 18 months ago. Who does that when they don’t know if it’s been valued or not or whether it’s a figure he’s plucked out of thin air? I really don’t understand why he doesn’t get some current valuations from estate agents and work from there. Bear in mind it’s a buyers’ market at the moment so I’d go and look for a bargain if it was me.

So you can afford the £350k valuation? If so go in strong and offer it (or something very close) to your landlord. If you haven't done so, no wonder he won't negotiate.

And suggesting I move all my things out of this house, and make good any marks from living here, to then buy it on the open market and move back is ludicrous.

Do you think he might not really trust your bone fides OP?

I mean according to you he may not even know that you have the finance in place and, that you've disputed his valuation, and, questioned his integrity, and, wanted to defer making an offer for another 4-5 months, (isn't that when the law changes on tenancy rights, hmmmm) and that offer would assumedly be what you feel it's worth then (with you as a sitting tenant I suppose) rather than equal to or more than the valuation he's said he received from a professional estate agent ~12 months ago, and lastly that you were entertaining the idea of hampering the section process to try to stop him exercising his legal right to break ; Yet he still wants to go to open market, rather than try to deal with you directly.... without knowing these all of these crucial ancillary facts....

Your LL seems very wise to me OP, sorry. Good luck with your house hunt.

And suggesting I move all my things out of this house, and make good any marks from living here, to then buy it on the open market and move back is ludicrous.

Do you think he might not really trust your bone fides OP?

I mean according to you he may not even know that you have the finance in place and, that you've disputed his valuation, and, questioned his integrity, and, wanted to defer making an offer for another 4-5 months, (isn't that when the law changes on tenancy rights, hmmmm) and that offer would assumedly be what you feel it's worth then (with you as a sitting tenant I suppose) rather than equal to or more than the valuation he's said he received from a professional estate agent ~12 months ago, and lastly that you were entertaining the idea of hampering the section process to try to stop him exercising his legal right to break ; Yet he still wants to go to open market, rather than try to deal with you directly.... without knowing these all of these crucial ancillary facts....

Your LL seems very wise to me OP, sorry. Good luck with your house hunt.

All these negotiation tips are pointless. He’s made a decision, let him have it.

Why fanny around trying to win his approval - it’s just another pick me dance!

Go see other houses. It’s a buyers market, you could find your dream home.

If your MIP offer allows to the value of this property, see a solicitor who will best advise on what to do. Just doing that shows your intention is serious.

You haven't said how you're in the position to buy after renting for so many years, nor why you obtained a MIP. Was it for this property?
Your LL sounds as though he considers it a pipe dream on your part.

Look, the OP has

1. Disputed the valuation the LL said he had got on the house over 1 year ago
2. Doubted the LL's integrity (possibly to his face)
3. Said she doesn't want to bid what she feels the house is worth for 4/5 months hence, cos she will be "in a better position to negotiate"
4. Openly entertained the idea that she would try up to mess up the section process to stop the LL exercising his legal right to get her to vacate the property.

The LL has invited her to bid on the (then Unoccupied & likely tarted up) house, through the open market bidding process, which is totally fair enough. If she really wanted the house at a fair price, then It's up to her to make a bid now that might justify him not thinking he may get a better price on the open market after going thru all the hassle & expenses of selling that way. But she hasn't done this, instead she's come on Mn to moan!?

Tbh, the LL is wise imho. There are red flags all over this. OP, if you really want the house make a fair bid for it now, and stop all the pissing around.