old alarm replacement advice please

[the phantom]

the wife's 6N Polo has been having issues with alarm that was presumably fitted when new (in 1995), particularly in triggering the central locking and preventing starting

so I'm thinking about getting the alarm replaced with a new unit, any advice on who/where the work might be done?  I know some alarm people are cowboys (not helped by the fact that all new cars come with alarms so the market is shrinking)

do I go to someone like Mongoose, an auto electrician, or just to a garage (Endeavour Motors from choice), any thoughts?