Thanks to all who replied. Here is an update on the now infamous Apollo.
I found two places on the web that will supply new exchangers for around the £370 mark.
As I said in my last reply, I would have a go at cleaning the exchanger. I talked to the local plumbing shop and they suggested using 'Spirit of salts'. It's the first time I've heard of it ! So after ripping out the exchanger, no great dramas there, I poured the stuff into the exchanger. That is lethal liquid. I was warned by the guys at the shop, no more than 15 minuets, there is a great danger of the joints being eaten away. After thoroughly rinsing out with copious amounts of water I replaced the unit, filled the system, checked for leaks and bled the system
Now comes the big switch on, I think the term is gobsmacked! In the six years I've been in the house I can't ever remember it being so quiet. I now always run the boiler on low. admittedly, around 30 odd seconds before the the boiler thermostat kicks in there is just the beginnings of kettling but it is a vast improvment on before.
Livy