Agree with Paul T on the length. 4.5m internal length would suggest a RIB roughly 5.4m LOA.
The transom has knees, which are not standard on a SR5.4 and it's been cut down with the exposed ply not made good. There's no bailer holes either.
The grab handles, fendering and lifelines aren't original Avon spec for a SR5.4. Bow area looks wrong too: SR5.4 has a single join where the port & starboard tube sections meet - this one has a wide central section. Not sure whether earlier boats were different? Toobs may have been worked on by Eurocraft - a lot of earlier Ospreys had that style of lifeline.
The lid on the Yam is 1988/9 ish.
Need to see some better pics.