Thanks everyone that's really helpful, appreciate it.
I was thinking of taking the gullet out like Skippys Mum suggested so I'll have a go at that and compare which one sits the best directly on his back, then I'll pop the best fit one in the saddle and check that again.
I thought the whole point of all these removable gullet saddles was that you change them on a quite regular basis as the horse changes shape throughout the seasons, so realistically getting the saddle fitter out 3 or 4 times a year isn't a very economical option for me.