In reply to Climbing Pieman:
Gulls, the basic UK 5:
- Black-headed
- Common - the least common of these 5 in respect of sightings but probably more numerous than GBBs
- Herring
- Lesser Black-backed
- Greater Black-backed
...are pretty easy to identify as adults - immatures can be confusing especially Herring and LBBs - I tend to ignore them. Get familiar with all these - difficulties tend to be (again ignoring immatures):
- Being sure it's a Common Gull when there's no Herring or LBB for size comparison, there's a wide variation in leg colour (as I recently found out).
- Being sure it's a GBB rather than dark subspecies of LBB when there's no Herring or LBB for size comparison, pink legs is the definitive bit but also GBBs look much meaner.
The only other "common" gull in the UK is the Kittiwake - but this is virtually always seen on cliffs or on/above the sea, virtually never inland. Easiest to see by simply going to places where they nest. No idea about your locale up north but in N England St Bees Head (W) and Scarborough (E) are easy places to see them during nesting season.
That then leaves the "occasionals" & rarities - you've either got to be in the right place at the right time (Bottom Clinger seems to always be bumping into Mediterranean Gulls which I've yet to see, and I missed the recent Lancashire explosion of Little Gulls in transit), or have huge resources of patience to scan winter gull roosts (scope usually required).
Ivory, Glaucous and Iceland are rare visitors but probably seen every year somewhere in the UK.
Finally, what's a separate species - Yellow-legged and Caspian are I believe now considered separate species but previously they were considered sub-species of Herring Gulls - I believe there's some sort of "continuum" of these hybridising with Herring and also possibly LBB. I've seen pictures of some of these with positive IDs of e.g. 2nd year Caspian and I'm just "looks like a gull to me, how can you tell the difference from a 2nd year Herring" etc...
Good luck - enjoy.