I wasn't happy with the lack of control over detail in the previous attempts so I decided on a different technique. I sculpted a face in plasticine, made an open plaster mould and slush cast a thin latex mask. After dipping the upholstery foam head in latex, I grafted the mask on while the head was still wet. I don't think I have a career in plastic surgery, but it worked ok.
Mask, mould, sculpt.
Still a bit rough but getting nearer to what I envisaged.
I have to thank Jon for the photo backdrop technique. Cheers, pal :)