On Sun, 06 Mar 2016 13:26:33 +0000, Cursitor Doom wrote:

Under power, with frequency counter connected between T1 and T2 with
V1812 removed from circuit shows the PWM chip pulsing at 22.55kHz.

Sorry, ignore that; copied the wrong piece of paper. It should be
20.64kHz. (This is with the load connected.) I then tried again with V1812
re-inserted and got 20.62kHz. Apologies for the earlier error...