[Moppie]

Ok, it sounds like a fuel problem.

My first sugestion would be to replace the fuel filter. (I have no idea how, I dont even know what an 1989 reliant is.
(the only Reliant I know is the britih reliant schimtar with a Crysler v8)

Since you've had the igntion system looked at, it means the problem is most likly fuel starvation at high engine speeds.
If a new fuel filter doesn't fix it then you need to look at getting the injectors cleaned.
(I also assume you've replaced the air filter? and its had the timeing set etc)

I would also find another mechanic, to be charged $300 and the problem not fixed is robbery. If it happened over here the garage would be in trouble with the Consumers Institute, and could be fined.