Just had exactly that done with my Westy. The guy works all over the country, he`s in demand everywhere

the alarm is a Toad Ai606 which is Thatcham grade 1. He does a very neat job, respects the value of your motor, you can`t tell where he did the work on mine, it`s so well hidden and neat. Also fitted remote central locking at the same time so the alarm fob locks everything. (you will need a new rear lock mechanism for central locking, £35 from JK, PM for details of how to mod your sliding door lock to suit, it`s a five minute job and saves buying a central-locking type mechanism)
07985144166 / 07738818399
safe.n.sound@blueyonder.co.uk
Ask for Barry. Give him a shout, I can really recommend him, and he knows exactly how/where to fit the stuff now, as he`s just done mine

Keep it real.
Search first - ask second ;>}