Success of, and eligibility for, implants depend on many different things. As you had an accident you may have lost some of the bone where the teeth were knocked out. This can effect whether or not implants can be placed in these areas or not. Success rates are generally very high, although as with all dentistry, things can and do fail. £2000 is not far off the mark for an implant and subsequent restoration with a crown.
The crown could be coming loose for a number of reasons e.g. failure of the bond, decay underneath etc. The reason for its failure will have an effect on what treatment can then be carried out. This should all be discussed with your dentist though, as each case is individual- he has access to your x-rays etc.
Hope that helps!