Central Heating/Hot Water Valve not working


80s Housing Estate, probably original CH.
The most immediate problem is that the rotating valve that switches between CH & HW appears to be jamming. I'm surviving by turning it manually with a coin, and I notice there's an area of increased resistance at around 2 o'clock on the dial. It's may be relevant that this is a hard-water area.
The make is Switchmaster; I can't see a model number.
Second problem is immersion heater is dead.
Are either of these fixable by self (some knowledge of plumbing)? Any advice to give?
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http://content.honeywell.com/uk/homes/systems.htm
will tell you what set up you have. Hard water should not make a difference unless you have a small leak.
As for the immersion, do you have a multimeter etc and know how to use one. It should be easy to find the fault
Adam
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