How binding is a mortgage offer?

[BearBibble]

We're hoping to buy a property at auction, which we've never done before.
We've done all our due diligence in terms of getting a survey, advice from an architect and a builder (as it requires significant work doing), getting mortgage approval, and having a solicitor review the legal pack.
The property is ex-council, originally sold under the Right to Buy scheme in the 80s. The solicitor has flagged up a potential issue in the title deeds associated with the original sale whereby the council has the right to reclaim a small area at the bottom of the garden at any point within the next 40 years. We wouldn't be able to charge for this - they can just take it back. The area is probably 1/7th of the length of the garden.
In addition, the seller has provided water and environmental searches, but no local search.
The solicitor is concerned that these may be problematic for the lender.
Given that we've already received a mortgage offer (not an agreement in principle) how much of a problem is this likely to be? Is the offer basically binding once it's been received or might the lender withdraw it based on these issues?
We have contacted our mortgage broker but he's on annual leave and won't be back until after the auction date.

The mortgage offer will always be dependent on the property meeting the mortgage company’s lending criteria. Have they done the valuation/survey yet? If not it could fall through based on that as that’s how they confirm whether the property meets their criteria or not. You can cal the mortgage company and tell them what you know, and they can probably give you an idea on the phone (or a callback) of whether there will be any problems.

The mortgage offer would go through your solicitor, and these aren't normally issued until the lender has undertaken a valuation, has that been done? Also auction property sales are normally required to complete within 28 days, will your lender meet that timescale?

Thanks both.

Our deposit is 20% of what our maximum bid would be, and the lender did a 'desktop valuation' based on that. They said if our deposit had been smaller then they would have done an in-person valuation / survey.
They've confirmed they can meet the completion timescale.
The offer came through our mortgage broker, not the solicitor, so I'm not sure if the lender and the solicitor have had any communication between themselves.
Good shout to call the lender. I was reluctant to do that in case it meant they had to start the whole application process again (which would mean the new offer didn't come through in time for the auction) but I suppose that's better than winning a bid on a property that we can't then get a mortgage for...
I suppose I was hoping someone would come on here and say "no, once you've had an offer they're obliged to honour it come what may"!

I'd explore some sort of bridging loan option too. I'm on various auction lists and seem to be bombarded by financing offers too. More specialised help might be useful?