Worcester 24i RSF Junior - Water Flow Switch Melted!!?

Hi, had a few probs recently wi boiler, new Gas Valve & new PCB within last 2 months seemed to have sorted it. Boiler is 8yrs Old.
However, 'bout 4wks ago noticed leaking from water flow sensor (SIKA brass 'T' joint, PN25). This is the connection from the domestic inlet pipe. Took part off, ptfe'd joints, put back together & seemed OK.
Within last week noticed that leaking again.When Hot Water is on the flow keeps the pipe cool, but when the HW is turned off and the the Central Heating kicks in the pipe gets really hot (steam comes from the joint where it is now leaking). The pipe only cools down when the hot water tap is turned on as there is a supply of cold water surging through the pipe.As soon as Hot Water turned off pipe gets red hot again & starts steaming. Tonight I took off the 'T' joint where the flow sensor connects to and the white plastic filter which is contained within the brass 'T' joint has melted!! Can't see that I don't have a problem as I'm sure that the Boiler shouldn't melt it's own parts.
I've put the part back on (minus the melted filter) and the boiler is somewhat tempermental. It sometimes gives hot water, sometimes not on demand. Fires up on demand then goes out after 5secs. I've turned power supply off then goes throught same sequence, fires up then turns off after 5secs. CH will only come on after a demand for Hot Water is made but doesn't stay on for long - 20 secs to 2 mins.
Why has my filter(I assume it is a filter) in the flow sensor melted and is the boiler being tempermental because I've taken out the melted filter.
Any advice will be greatly appreciated because I'm on the verge of shelling out a grand to have a new boiler fitted and I've much rather spend it on a holiday.
Thanks