Thanks beneagles and discomania.
Its a 2009 Defender 90XS - the 2.4l Ford Transit engine - brand new (i know probably lesson learned). They misdiagnosed a problem with an idler and replaced parts twice before they realised that it might be "wax on a belt". I've queried a deep noise which comes from the passenger side of the engine when i switch off - they put this down to the EGR valve working and "perfectly normal", but they dont look convcinced themselves.
To be honest, I've no idea and I'm quite happy to pay an independant engineer to give it the once over.
Coolhand