Thanks b40rt, yes the frame fits with Martin's description of the 1978 model. The DVLA have the bike registered as being red too although at present it's yellow with home made side panels so someone must've tried to emulate the later look at some point. It was like this when I first owned it although it used to have the proper Marzocchi forks and they've since been swapped for some random forks off a trail bike ?
So, currently looking for a pair of forks too... ?