Overflow in tank suggests system is starting to sludge up.
Boiler cutting out after 30-40 seconds suggests system sludging up or pump not working.
Water very hot suggests system sludging up or pump not working.
I think you have a blockage in the system - cheapest solution
First check pump is working ( i think it is as you say overfolwing into tank )
Second Turn off boiler then drain whole system (attach hose and open drain off tap so a fast flow is obtained) look at colour of water coming out - if very dirty you may have to do the next step a couple of times.
Close drain tap - refill sysyem - bleed all rads - run system for half an hour (if it will) - Turn off boiler - drain system again - check water colour.
Third - if still no joy then try a chemical treatment (Sentinel x400 Sludge remover) May take 4 or 5 weeks to work properly you add this to the tank then fill the system and run it - after 4 or 5 weeks you drain system again and refill with fresh water - run for half an hour - re-drain and refill not forgetting to add an inhibitor chemical ( Fernox Mb1 Sentinel x100 or similar)
Hopefully that will sort it out for you - 1 other thing when draining the system tie up the ball valve or turn off water and starting at the top open the bleed valves on the radiators to let air into the system so all water is removed. DON'T forget to close bleed valves before refilling - then starting at the bottom bleed air out.
Fourth (LAST RESORT) Pay a small fortune to British Gas (about £500) to come and power flush the system.
Stewiesnake, November 2008
