If the two accompanying gulls are Black-headed (as they appear to be) then on size, form and bill shape I would say that your bird is an immature Herring or Lesser Black-backed Gull, but plumage detail is not very clear. I would stick my neck out and say 2nd winter Herring.
I am currently using a 70-300mm lens with a 1.4 converter. The trouble is that the lens maximum aperature is 5.6, add another stop for the converter and there is just not enough range. Auto focussing seems to be an issue too with this combination. Absolutely fine with a 12-60 lens with motor drive. Looking at replacing the telephoto gear as I prefer action shots and I don't think my current gear is up to it.
The pale inner window on the primaries can be seen well in your picture - which makes it a Herring gull.