I have never knowingly seen a yellow-legged gull. Indeed, as I started to write these notes I realised that I knew virtually nothing about them other than they look very much like a herring gull. I could find little about them in Britain other than in the excellent RSPB Handbook of British Birds which rarely fails to deliver at times like this! With their help I can now tell you that the yellow-legged gull nests mainly along both sides of the south western Mediterranean coast in Africa and Spain but has, in recent years, spread its range further north. After breeding they disperse northwards and increasing numbers are now being seen along the south coast of England. Unlike herring gulls it tends to be a somewhat solitary bird choosing to feed on the tide line on its own. It also prefers the company of lesser black-backed gulls that herring gulls.
The weekly reports chart seems to bear out what the RSPB Handbook says. Occasional reports come from the early weeks of the year but in week 28 at the end of July reports increase and remain higher during August, September and October. Then from week 44 at the start on November the numbers fall back to being very sporadic. Where they go in November I have no idea but perhaps they return further south as the weather deteriorates?
All the reports in Dorset, as you might expect from a gull species, come from coastal sites with records from Christchurch Harbour, Poole Harbour, Portland and the Fleet. Ferrybridge and Radipole seem hot-spots for them with Lytchett Bay also seeing more than most.
Puting yellow-legged gull on your Dorset list is not going to be easy! First you have to learn to tell them apart from herring gulls and then you need to be in the right place at the right time. Good luck with that!