Unfortunately, phpbb3's cookie settings cannot work across a forum with two separate names ie. isisadventure.co.uk an isisadventure.com. The only way to currently do it is to have one address redirected to the other. Since I use the .co.uk address and have no problems, I suspect that the people who are currently having problems are using the .com address.
Ideally, admin should redirect there .com address to the .co.uk address. However, I suspect that they have shared webspace across the two domains, so this may not be possible.
For anyone having problems, just stop using the .com altogether. As far as Im aware, everything is available on the .co.uk address too, and seems to work properly.