I've messaged Dave via the service and said I would happily pay him, no reply yet.
I had a thought, if I export the dat as a .txt, then change the encrypted private key to a new one that I know, then export to a .dat
Would that work? Like overwriting the original encrypted private key?
That wouldn't work. If you changed the encrypted private key to a new private key with a new password, you would lose access to your old encrypted private key holding the 10BTC worth.
One thing you can do though if you can't figure out the password is keep the wallet.dat file safely backed up for a long time - I'm sure that in 10 or 15 years it'll be much easier to crack a 30-40 character password than it is now with faster computing, plus there's always a chance that the encryption algorithm for Bitcoin-Qt will be cracked. That'll definitely be a long time from now, but it might be worth it.