He first became interested in haemophilia as a trainee pathologist in 1970 when attempting to treat a patient with haemophilia A whose joint bleed failed to respond to the usual dose of cryoprecipitate. This failure proved to be due to a newly developed inhibitor. During further training in Cardiff under Arthur Bloom he was drawn to the cryptic relationship of von Willebrand disease and haemophilia as a key to unlocking the secrets of factor VIII, a theme he pursued under Leon Hoyer in Connecticut (1976/7) then in London at The Royal Free Hospital where he was appointed successor to Katharine Dormandy. By 1982 total purification of factor VIII using monoclonal antibodies was achieved by his team in sufficient quantity for rotein sequencing, leading to cloning and synthesis in collaboration with Genentech in 1984. Using the genetic tools created by this large scale effort he worked on the mutations causing haemophilia A, linkage analysis for carrier detection and antenatal diagnosis and the site of synthesis. From 1987 to 2005 he led an MRC haemostasis research group which achieved many advances in the area of rare bleeding disorders including combined factor V and VIII deficiency, factor VII deficiency, factor XI deficiency and VKORC1 deficiency. Moving back to the Katharine Dormandy centre in 2006 he was principle investigator for the first successful trial of gene therapy for haemophilia B in collaboration with Amit Nathwani. He continues to work on developing haemophilia gene therapy trials for haemophilia A and factor VII deficiency.