I ordered some Iosso Mold and Mildew cleaner off of Amazon. It was expensive at around £30 a tub but it was posted from the US. It's worked really well so far and once I dunked the tent in a barrel in the Iosso and hot water overnight I just had to wipe off the mold with a sponge, some of the heavier marked areas required a little scrubbing. In some places I noticed that the waterproof membrane was peeling a little where I was scrubbing, as the mold had penetrated it and I will need to re-waterpoof these bits. But this is a small price to pay for saving such an expensive tent. Just need to treat the inner section of the tent and the more heavily moldy door area and it'll be done!
Tried treating the inner cotton tent with the Iosso but this didn't really make any difference, even with scrubbing. So after a bit of googling I found a product called Borax, which apparently will remove mold from cotton, the only trouble is it was handily discontinued. Luckily I discovered Robert Dyas do a product called Borax Substitute, which is very similar and only cost £1.60.
So I washed the tent with the Borax substitute, which seemed to help only a bit, then I followed the instructions on the box to treat heavy staining and made a paste out of the Borax Substitute, Bicarbonate of Soda, Salt and water and then got a sponge and basically scrubbed at the tent until it was off (took probably about 5 hours on and off). Finally I washed the tent to clean off the paste. It looks to have come out really well.
Just need a few days of nice weather to get the tent up and check it all over.
Finally managed to get the tent put up (had to do it in someones elses garden as mine wasn't large enough) and to my surprise all of the mold has now gone from the tent and also the cotton inner. The only remnent is a bit of yellowing on the plastic windows, but I can live with this. I re-water proofed the tent using Nikwax Tent & Gear Solarproof, can't wait to test it out over the summer!
The products I used for the process were:
Iosso Mold & Mildew Remover (which I placed in a large barrel and soaked the main tent in overnight, I was then able to wipe off the mold, though some areas did require a little scrubbing),
Borax Substitute (which I washed the tent cotton inner in, and then scrubbed it with salt, bi-carbonate of soda and water. Definately the most time consuming part of sorting the tent out),
Nikwax Tent & Gear Solarproof (For re-waterproofing the tent).
I also used Milton Disinfectant to try and remove the mold from the outer tent but it didn't work.