Flood due to boiler ???

A few weeks ago we noticed warm/hot water pouring out of the eaves of our house, upon inspection we found that the cold water tank in the loft was overflowing so , the excess water was being driven away through the overflow piping of the tank, however the water was warm/hot, the system comprises of a Potterton Suprima 100L boiler (yes already experienced lockout , so have replaced PCB, didn't realise though that this was a common scenario, won't buy Potterton again), a large cold water tank, an immersion heater and a header tank.
The bull valve and cock were all ok, no problems, so drained system using the cold taps on the baths, thought that it was a one off, wrong about a week later it happened again, done the same thing, drained etc all ok, but this time turned down the room thermostat and reset the water and the heater times which were on full blast everyday.Great a week went past then my missus walked into a tropical rain forest, the colkd water tank had overflowed and had come over the edge of the tank, she god bless her contacted a plumber that we both know , and he came round quite sharpish, drained the system again, found that the water level was too high in the tank , as the immersion tank overflow was being immersed into the water of the tank, found plastic swarf in the ovrflow pipe of the water tank, where a filter had been fitted and a small piece of cellophane aswell as the swarf had blocked the overflow, so removed the filter, also in addition to this the overflow pipe that goes into the tank was at a 90 degreee angle not at a 180 degree angle.
OK , yes at last the fault has been remedied after causing 7to8 K worth of damage, insurance job, however a couple of days later we again started to see warm/hot water coming out of the eaves, went straight to the tank again the tanks cold water was warm/hot and water was flowing out of the overflow, drained again and called the plumber back, a mystyfied plumber checked the immersion tanks thjermostat the motor that controls the water flow , but to no avail, spoke to a boiler man who I know has been in the trade all his life and is very experienced , couldn't think of anything, apart from to check all connections of the PCB that had previusly been fitted some 12 months ago. When I checked the connections I noticed a that there are two tubes coming from what I think is the thermostat on he boiler , which is located behind the PCB but towards the back of the boiler , one tube goes directly into the boiler part ie where the water is heated and the other tube seems to rest on a copper pipe at the rear, now this tube looks like it has a metal coupler that is half inserted into the tube but as I say just rests on this copper pipe. Hope you haven't dropped off yet , and that i have explained myself well.Probably not as I am a novice at boilers. Hope to hear from you all soon. many thanks