any descent builder should be able to quote accurately from the planning drawings (he can get quantities from this) and a specification from you (what type ofdoor and frame you want, floor finish (screed or concrete, electrics, roof tiles, guttering etc)
search for a builder by recommendation from neighbours, friends etc
£750 seems alot unless it includes the fees (unlikely) engineers calculations etc
another though worth considering (if you are having a pitched roof) get it constructed using "cut timbers" rather than trusses with floor joist instead ofceiling joists – this would make extending above the garage at later date much easierand cheaper. You would most likely need to have the garage walls built in cavity construction or 9" solid brickwork