It's the DIAPHRAGM PRESS/DIFF VALVE product codes 439519 from the plumb centre, BAXI code 248066 cost is £6.44 from the plumb centre, I've had three boards fitted numerous other parts fitted.
Found this info on another forum done it myself only takes 10mins.
A plumber quoted another £400, I've spent over £1200 on repairs in the last two years. All for the want of a £6 part.
julie
October 2009