Ravenheat combi fault: No hot water without CH

Hi all,
Can't get hot water from out 820/20T unless the CH is on too. When set to DHW only, main burner doesn't ignite.
The fault-finding flowchart in the manual says:
Are microswitches on shunt valve being operated mechanically?
If NO, check/replace 3-way microswitch. If NO, is flow rate > 3l/min?
If NO, check filter flow. If YES, replace 3-way valve.
Firstly, are the "shunt valve" and the "3-way valve" the same thing?
Secondly, one of those first two NO's should be a YES. Should that be the second one (ie. If YES, is flow rate > 3l/min?) ?
Thirdly, how do you check whether the switch is being operated mechanically? It's on the back of the 3-way valve which is at the back of the boiler behind a rat's nest of pipes and wires. What should I expect it to do and is it possible to check without removing the whole valve first? And if I remove the valve first, will it still do what it's supposed to do with no through-flow?
Fourthly, I got the last 3-way valve from Travis Perkins a few years ago as I had an account at the time, but any recommended alternative suppliers?
Thanks for any help,
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