Thankyou so much to all those who have given me input. It's helped me narrow down what I think the issue is.
Unfortunately I don't know any local hams, let alone any that would be willing to join me on an activation.
I have done some checks, I can open a 2m fm repeater just fine so I've definitely got a carrier.
Power output flutuates on ssb if I speak into the mic when keyed up so the microphone is not an issue. The ft-817 was bought new a month or so ago so I'm hesistant to perform an modifications on the mic as of yet though it is something I may do in the future.
The power level indicator is flashing on battery and not visible when running of an external power source so I should be getting 5w output.
I've been getting (poor) signal reports on 40m WSPR from as far as the Canary islands when running through the tuner and a 4:1 unun into a random wire strung up down the garden.
This is leading me to think that the Sotabeams bandspringer is letting me down, it is essentially a random wire and counterpoise so I'm losing a lot of RF in the tuner.
My next step is going to be to construct a linked dipole for 40, 30, 20 and 10 meters, hopefully a reasonant antenna system will resolve my issue.
Does anyone think this is a viable combination of bands for the current conditions? Which bands would people suggest?
I'm also going to construct a ladderline slimjim for 2m, I've already build a 2m tapemeasure yagi which performs suprisingly well but it's a little unwieldy to carry to an activation site.
Again thanks for all the input so far!