I have an Alpha combi boiler which hasn't worked for 5 of its 9 yr life.  Pressure keeps dropping. Central heating been okay but hot water is a no no!  British Gas threw me off their Service Contract last year because they could not get it to work consistently and had practically re-built it.  I was on the contract for 3 yrs.  They quoted me £6,500 for new boiler and re-piping.  (Said the pipes would have to run down the outside of the wall!). I have decided to buy a new boiler but someone said it may not be the boiler but the wrong type of plastic piping.  How do I know?  I rang the Building Regulations dept at local council and they had never heard of plastic piping which may be porous. They told me to get water pressure checked.  Three of my neighbours have the same problem though and I don't want to fit a new boiler which will still not work.   Any experts out there on plastic piping?

Have scratched off the paint where it goes into the radiator.  It is plain grey - no markings at all.  Is there anywhere else I can check?

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I think that's the clue. Perhaps all three houses are fed from the small bore pipe. See if your pressure/flow drops when they turn on their taps. You should get the pressure AND flow rate checked. It they are on the low side then perhaps consider switching from a combi to conventional boiler with h/w cylinder, header tank and pump (or one of the other systems that store water to provide peak demand).