Please could someone help me understand how this works? We've spoken to a couple of mortgage advisors and are even more confused than before! Apologies if I'm being dim, but I'm rather number blind and am finding this very difficult to understand.
Basic situation:
We own a house we bought 5 years ago for C£230k, which is likely to sell (v quickly, desirable) for C£320k. Mortgaged for £200k.
When we bought we were both employees, earning C£25K each, to a household total of £50K (ish) pa.
Now I'm a SAHM, and DH is self employed earning approx £60k pa, so our household income has in fact risen.
We want to buy a house for about £350k, which needs serious work doing to it so want to borrow an extra £50k (so DH says, but we could prob do just the bare essentials for less and live in DIY hell for a few years).
To move our mortgage over to the new property the mortgage co say we need a deposit of minimum 5% (= £17,500 to borrow £332500), and due to the change of employment it would be treated as a new application, but they'll accept two years of proof of income (via accountants certificate).
They won't lend any more than the value of the house.
Other mortgage advisors have said that as DH is self employed he would need to provide them with contracts proving current work, but his work can be on a rolling weekly basis or for a month, or even daily - so this won't work.
We don't have enough savings to cover the deposit, just enough for the conveyancing etc.
So how do we do it? Does everyone have £15-30k saved up before they move, or does the deposit go up the chain once our house has sold?
My theory is we sell our £230k house for £320k, making £90k 'profit".
Pay 5% deposit of £17.5k, leaving £72.5k 'profit'.
Borrow a further £132500 (?) on top of existing mortgage, making a total of £332500 borrowed, to new house value of £350k.
Purchase house for goal of £350k inc deposit
Then use the £72.5k to do the works required to make it liveable, ideally this would come in under 40k.
The purchase price may well be more than £350k, these are just guesstimates at the moment (house being sold is vacant and we currently have 'first refusal').
Does that work, or even make sense? 
Any replies appreciated. 