ideal boiler lxff?

When you say 'top it up from the tank up stairs' do you mean filling it via a filling loop?
Either way, you have a leak on the boiler/system somewhere.
On the boiler, check the pipe outside, if it is leaking when you have filled the pressure up, the prv is knackered.
This could also be caused by something else, when you have the boiler topped up and running, does the system pressure increase significantly when the temprature rises? This would indicate the expansion vessel needs re-pressurizing which would also tie-in with the prv being blown.
Also check system pipework and rad valves etc for leaks.

About your radiators, warm at the top cold at the bottom, check your thermostat is up at a decent level to give enough heat to the radiators, if it is and they are still warm/cold. This would indicate a sludge problem in your system which would need cleaning. The likelihood of sludge being present in the system is in direct correlation with constantly adding new water into the system, the new water will be aggresive against the metals in the system causing a sludge problem.

A bit long winded, but it may point you in the right direction

Carefree
July 2010