Yeh thats it! There is no copy of the letter from the bank they just called him last night to say they approve - he still does not have the letter so in my mind they can still change theirs.
No, they can't. The finance clause is solely for the purchaser's protection, not the seller's. The seller has no option to pull out for fear that you can't finance; it's exclusively an out for the purchaser. The default position is that if you don't do anything by the time set for expiry of the finance clause, that the contract becomes binding.
The default position is that the contract becomes unconditional; the time is a deadline for the purchaser (only) to notify their intention to pull out of the contract due to lack of finance. If you're confident that you'll obtain finance, then notify that you're happy to go unconditional. The vendor doesn't get a say in it!
It's actually if you're declined for finance, and wanting to exercise your right to cancel the contract, that verification of the lender's position is important. Nothing is needed if finance is approved.