... and have come to the tentative conclusion that the early flowering white forms of series Pontica are Rhododendron x sochadzeae (Rh. caucasicum x ponticum).
I haven't come with up an identification for the even earlier pink flowering forms, nor for the red flowering forms.
I have a white on one on the lawn. The one in the photo is yellow, it might not be showing up terribly well. The white one came in weeks ago and it's lost its flowers already.
... (depending on what you mean by yellow) then its unlikely to be Rhododendron ponticum, which is purple, shading to pink, red or violet, with (IIRC) a simultaneously flowering white form.
Lat/Lng: 53.8, -7.1
OS grid ref: SA7154