Typical response : It sounds like the fitter has dislodged some wiring somewhere when fitting the HG.
I had an alternator fitted by a **** who failed to spot a blown fuse, I ended up taking him to court for a refund.
I would do one of two options, get a good motor electrician to try and diagnose the problem, or try and get a garage with a system computer to trying and locate the fault.
Push comes to shove, get LR to put their computer on it, it might cost a few quid but peace of mind is worth it.