powrmatic pgu-75-C-1

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Can any-one give me some advice on the Fan stat of a Powrmatic PGU-75-C-1
According to Powrmatic this stat has been replaced by a timer fan contoler at a cost of £75:00 + Vat + delivery.
The part number of the original device is 142403593 and is a simple 4 wire bulkhead mounting unit. The replacement unit is part number 142403605. There is no image available for this so i'm not sure what it looks like or how it interfaces with the 4 wires on the orinal stat. On the original stat two of the wire feed a miniture ceramic disc heater in the stat body and the two remaining wires are across a bi-metal switch which operates the blowing fan.