combi 80 HW heating rads, lockout

Hi, I have a Potterton combi 80 with the following problems: the hot water flow is heating the radiators when the CH is off, I would assume this would be the diverter valve/wax capsule head, what's the best way to test this?
Also the boiler locks out with increasing frequency, I went through the fault finding checklist in the manual and it pointed to the overheat thermostat which I have replaced, this worked for about 3 days. It seems to happen when the boiler has been running for a while with CH on and then after hot water has been used and stopped when HW is requested again
It seems worse when the casing is on the boiler (I left it off for 2 days while testing it) so I assume it's still overheating somehow?
The main pcb was replaced about 6 months ago.
I'm wondering if it's the fan that's maybe not working as it should, again what's the best testing proceedure?
Many thanks
Will
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Fix the diverter valve first. It may be that the valve has failed in a way that prevents effective pump overrun, causing the overheat to occur. It might not, of course, but it's a possibility, especially if you have TRVs on all your radiators.
Christian.
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ahh ok they all have TRV's so i'll start with the diverter, many thanks!

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