Im covered by my bank, for no extra charge
but as a heating engineer myself i would diagnose the problem and tell the bloke what was wrong so he can supply the part for nothing :-)
this is no help to anyone might i add...
depends on how much you wanna pay really?if your system can cope with pumping the underfloor and not starve the rest of your system, underfloor can be pricey! i personally would stick a rad in there but its personal choice
my educated guess would be the pipes arnt clipped propperly/at all or u have a loose washer somewere could be anywere...
has this been happening forever? is it from every tap eg kitchen and bathroom?
its likely you will need a new pressure relief valve if water is coming out of there, there relatively inexpensive and not difficult to change in most cases.