I have previously installed interlinked smoke alarms in 2005 (not wireless) i now need to add a heat alarm to comply with regulations can I do this with my existing set up or do I have to change all my alarms to wireless? 

Thanks

If you installed wired smoke alarms in 2005 You should change ALL of them as they only have a 10 year life.

If it is easy to install a new 3 core and earth cable from one detector to your new Heat alarm, then I would do that as it will be cheaper than all new radio alarms, but if it is not easy then yes, change them all to radio interlinked alarms.