Was the error message just telling me that the battery was low even though there was power available.

Thats what they normally do, the control panel battery needs to be replaced at least every 5 years regardless of what condition it appears to be in (it could still pass all the normal tests). In some cases where the control panel is in a warm enviroment such as an airing cupboard that timescale can be reduced to a few years.

In all my years on service i have never had to replace a TX because it had gone faulty.

Thanks for all your help. I dont have the original tx, what harm will it do if i leave the other one in. Also if I need a new tx can I buy them anywhere.

Whats the VA rating of the new one you have fitted..?

I think its 7va

Expect problems then until that`s replaced.

Look for something at least 20VA if your staying with 12VAC TX, or the voltage drop under load will cause faults.

Ideally find a replacement of 18 Volts and at least 18VA rating.

Can I buy the tx at an alarm wholesaler

I doubt it, best bet is Maplins or RS.

thanks a lot for all your advice.

That`s why were here