Just as a postscript, I eventually located documentation for this boiler on Baxi Partsarena.com as follows:
Go to baxi/partsareana website... then go Main... goWater heaters... go Medway... go Basic later production... go Installation instructions... go Mersey, Medway automatic & Trent... and there it is!.
Heat input 30.27KW, heat output 22.7KW, raises 12.5L/min by 25C. Minimum operating head if connected to static tank 2.29M, max pressure 13 bar. Also interesting servicing stuff like how to descale and tweak burner fire up times. Shame this document wasn't waiting by the gas meter for me when I moved in many years ago...
DIYSOS