Sounds more like the Allon White version to me - the Simmonds version has over centre catches on each end of the header rail. We had the Allon White version on our 1985 2 seater. IIRC it depends on the hood being in tension to work correctly. If my assumptions are correct, you might want to check the rear of the hood to ensure that everything is still tight - on out 2 seater, the frame that was used at the rear could easily be bent out of shape which causes the hood to slacken. Need to see pictures of both ends to be sure though.
If it is the Allon White version, worth a phone call to see if you can speak to their head trimmer (Shaun) who will be able to offer advice.
