Frame looks to be built with Bocama Pro lugs. Campag short dropouts began to appear around the late 1970s. Cable guides on top tube are from around the same time and the Campag pattern cable guide on the top of bottom bracket shell would fit the late 1970s/early 1980s. The 26.6mm seat tube diameter would point to a French frame. Are the bottom bracket threads and fork steerer tube British Standard Cycle? Mercier, Motobecane and Gitane were imported into the UK around this time. Gitane had a following because of 'the badger' Bernard Hinault's success.
Thanks for the input,the threads are British and i did think Gitane because they made a 531 with British threads.Its the serial number thats the killer,thats why i thought it could be a Diamant (Belgium) as they had there frames made by lots of smaller frame builders then badged them up,thats according to what i found on the web.Does 26.6 denote a certain tube make?