For anyone going to have a look for the Thayer's Gull in yellow-belly country, here's my thoughts on viewing based on my time there yesterday. The original site is along the minor road from the 'roundabout junction' of the A15/M180 to the village of Elsham. There's a small electric building/station/can'treallythinkoftherightword halfway along which is opposite the ploughed field where the bird had been found. The viewing from here is pretty limited as the gulls settle just over the brow. In my opinion this is where the bird feeds for most of the day it's just that you'll realistically only get brief flight views from here unless you're very lucky. From here, the bird goes to bathe and generally chill out with his home boys on an area within Elsham Hall itself. Follow the road from the original site through the village until you reach a T junction. Go left. Left I said. Then go left into Elsham Hall and park ABSOLUTELY ANYWHERE leaving your car or moped strewn across the road or even leave it rammed into a farmers gate like I did. The chill out area can be viewed from here, in fact set your scope up on the roof of your car (or moped) and view from atop your vehicle. Atop? Good word. The bird came into this area 2 or 3 times yesterday spending 20 minutes or so at a time. This is, realistically the best place to get it on the deck but will require some patience.
In summary, you have two options, 1 stake out the original site and get lucky (also, if it's found at the chill out zone you should still have time to connect from here) or 2 stake out the chill out zone and wait and if it does show, it shows well.
There. Just my opinions. And not one swear word. If this helps just one person, I will have a warm glow for the whole Easter period, safe in the knowledge that I have helped a fellow human in my quest for a beautiful world.
Edit: the bird is missing a primary on the right plus two central tail feathers which allows you to get onto suspects even at height.