hi I have a potterton back boiler, I believe its about 20 years old according the to the last engineer. it gets serviced every year. its basically makes a lot of rattling noises, which I got told is due to age and now its struggling to start up it trys about 5 times before it actually works and then about 10-15 mins later it does it all again? any ideas on solving or is it ready for a new one?