Four bites/attempted bites in 3 weeks is alot.
Were you given any history with the dog?
Like Lankeela says I think the RSPCA should be the ones to pick up the bill for any behaviourist etc. I too am surprised that they rehomed the dog to be honest.
Some behaviourists can be VERY pricy (I have heard of 300 pounds being charged). Anyone can set up as a behaviourist so be careful where you spend your money.
A good site for advice is
http://ttlntl.co.uk/forum - you can join the forum (free). There are a number of experienced behaviourists/trainers on there that can and will give you good advice. It is run by Gill White who used to be a behaviourist for the Blue Cross.