The model has now been glossed varnished using Tamiya X-22 Clear followed by an overspray of Mr Hobby Self Levelling Thinners;

As I thought, the varnish has darkened it down a bit and the colour now looks closer to the Ospreys in the reference pics I have. I WOULD say at this point that the Osprey is ready for decals, unfortunately a crack has appeared around the tail;

This now requires filling and sanding which is a pain in the arse as I really wanted the model done. I thought I heard a cracking sound when removing the masking tape but didn’t see anything obvious until today. Dammit!
Looking at the area affected it would appear that this is a weak spot on the kit as the plastic flexes, especially where the cargo ramp meets the upper fuselage and something you need to be aware of if building the kit as it will need strengthening internally. For this I would suggest putting in a bulkhead in the rear fuselage, something to remember if I ever build another.