Water Tanks Overflowing

Thanks guys, thats my weekend taken care of then!!

you must replace the cylinder before adding any chemicals.

When the boiler is running the overflow rate of the small tank increases due to the water level rising, as it happens when the boiler is operational is this a sure sign that the cylinder is faulty?

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The overflow rate increases because the boiler is heating up the water. As it heats it expands and rises up the feed/expansion pipe. The clue is in the name given to the pipe. This has no bearing on JohnD's diagnosis with which I agree completely.

Here's another question. Have a close look at those mixer taps. Mains fed mixers, as used in kitchens, have two holes in the end of the spout, one each for hot and cold. They aren't really mixers at all. Does the bathroom mixer also have two holes? A single hole mixer should not have tank fed hot and mains fed cold.

you won't get much change out of £500 and cylinder changes now come under part L regs
unless you pic a cowboy plumber who don't have L regs cert

Typical, I used to have a British Gas Homecare service agreement that I canceled in May of this year. I had it for five years and never claimed, it seemed a waste of money at the time so I canceled it, oh how I wish I still had it.