Guardtec ?

I have old guardtec that has been trouble free for many years until now.I have read on here and other forums that the battery could be at fault.I have checked the battery connected and disconnected and have a reading of around 11 volts.My main worry is the battery doesn't appear to be getting any charging volts when it is connected.Iam thinking that the transformer has died as iam getting no output reading.How much is a new transformer likely to cost and where can i get one from.Or should I simply replace the alarm utilising my bell pirs and both door magnetic contacts.If i go for a new alarm and keypad then what should i buy that will connect up to everything else easily.