Looks like one of the Pride and Clarke 250 Panthers which might well be an impressed bike from the begining of the war.
You will have better luck chasing one of those than the New Imperial.
The little Panther was scorned for years by the snobs of the VMCC, who labelled it grey porridge, which is typical of the VMCC crew in the 1970s especially, but they were not a bad little bike, probably the cheapest 250 on the market, very cost cut to the bone, and all were sold by Pride and Clarke in London who I think bought the entire output of P&M; they'd go on for ever, over 100mpg, and took an amazing amount of abuse and neglect from men whose only transport they were back then.