Thanks in advance to to everyone for their advice/help, here's the situation now:
I still have the overflowing water from the cold water supply tank, and I've had a new float valve fitted, but the tank is continuing to fill and overflow(but not from the valve inlet).......the water in the tank is almost tepid at times. What has become noticeable is that the overflow will still run , even if it's only a trickle, hours after both the C/H and Hot Water is switched off at the boiler(cylinder thermostat checked and not faulty)......plus the hot water takes forever to get warm at the bathroom tap, much longer than previously.....there's also the problem of a reducing supply of hot water, it eventually runs to a trickle and I have to attach the washing machine hose to the hot tap to get a decent flow again (but that doesn't last for long after running the hot water).......had plumber/C/H engineer in and they are puzzled so you can imagine how worried I am becoming at this unsolveable? problem.......C'mon guys please please HELP!!!!!
p.s The central heating appears to be working fine with no problem getting radiators warmed up.