I have been given a dinghy and I believe it to be a Merlin Rocket, as all the photos I have seen and all my reference manuals point in that direction. It is in a sorry state but I believe repairable, or I'll die trying.

I have crawled all over the hull to try and find a makers plate and its registered number. Would it be located in a rather obscure place on the hull?

Carved into the centre thwart or maybe in the hog (inner keel plank). If there isn't a number anywhere it was never measured and may not belong to any class. Are you convinced it is a Merlin Rocket. Is it exactly 14ft long? There were hundreds of clinker built Sailing Dinghies within and outside the popular International 14ft, Merlin Rocket and National 12ft classes. Some were one-designs, designed sailed only by one club.
Give us a few more clues and maybe a photo or two and we might be able to identify it.