Bongo Fury

Thinking of getting a leisure battery installed but wouldn't attempt it myself. Does anyone know how much a garage might charge for this and is it something a standard garage would do?

Your standard Garage might attempt this it is not difficult IF they understand what they doing. However we have seen MANY examples of not only poor installations but seriously dangerous ones
You would be better talking to an Auto Electrician who knows Bongo's. There are also kits available which are designed for the Bongo specifically.

There are lots of companies that do campervan conversions and they would probably be able to help. Put your location in your profile and someone will be able to recommend somewhere in your area.