Glowworm Fuelsaver MK2 Pump overrun

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I have an old Glowworm Fuelsaver MK2 in a Y plan installation.
It's still working ok apart from an issue with the pump overrun...

As of 2/3 weeks ago, when the room stat switches off the boiler, there is sometimes a thump in the vent/expansion pipe as hot water / steam is pushed up into the F&E tank. It doesn't happen every time. It suggests an intermittent problem with the pump overrun.

I had a similar problem some years ago and replaced the boiler thermostat which cured the fault.

This time, to help diagnose the fault, I connected two neon lamps into the terminal box.
One shows when the pump is on, the other shows when the switched live to the boiler is on.

This is what the lamps indicate:-
When the boiler is running normally, both neons are on as you would expect.
When the room stat reaches temperature, both neons turn off together. This happens immediately.
After a short delay of maybe 3/4 seconds, the pump neon switches back on.
(This is when the changeover microswitch in boiler thermostat connects the pump to permanent live rather than switched live).
The pump then runs for about 25 seconds and switches off when the water temperature in the heat exchanger drops and the changeover microswitch connects the pump back to the switched live.

I'm no CH expert, but twenty five seconds seems like quite a short delay.
On this particular test, there was no thumping noise in the vent pipe.

Question1: Is the short delay before the pump turns back on due to the movement of the 3 port valve?

Question 2: Is 25 seconds pump overrun delay too short?
Obviously the time delay depends on the temperature of the water in the heat exchanger and how long it takes to cool.
I did have the boiler temperature control set low when I ran the test (position '1' on the dial that goes from '1' to '5').
The pump was on the slowest speed (That's always been fine in the past).

The boiler thermostat is a Ranco C77P0112 (glowworm part no 416189).
These are now obsolete and it's impossible to find a replacement.
As far as I can tell, there is no way to adjust the pump overrun temperature set-point on the thermostat. The adjustment screws appear to use thread lock to prevent tampering.

Question 3: Does anyone know if there is an alternative replacement for the C77P0112 thermostat that will fit?
I've been told that all the Ranco C77 range are similar. The main difference seems to be the size and shape of the bulb.

Please no advice about replacing the boiler with a modern, more efficient one. I don't want to do that until next year when the weather is warmer!
 
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A permanent live is required to spade terminal 9 of the Ranco stat.
 

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