We have got a customer that has a very old fire alarm system installed and when we carried out a recent service it was noted that the system did not comply with the latest British standards and we have recommended the site be up graded. For example the existing cable is not fire rated and also has no detection throughout. The customer has asked that because the install is over 20 years old, would we not compare it to the standards from when it was installed. I said that we would always work to the latest standards and therefore we would need to install a complete new system but i was just after some clarity to be sure?

It all depends on the fire risk assessment, what does it say about the fire alarm and what category should be installed. The final decision is up to the Responsible Person whether or not to accept it but from what you say it sounds that is should be upgraded and I cannot see the enforcing authority accepting it neither.

If the RP accepts the situation you should make your opinion very clear to to the RP, preferably in writing.