Hi Phil, no doubt a number of Hudds Birders are thinking 'I Wish I was There' rather than here with not much to look at!
Anyhow good on you, first of all you are more than likely, based on your location and time of year either looking at Red-throated Diver or Common Guillemot.
Size and shape of the bird is the first thing to look at, although looking through a scope at a distant bird can be tricky whatever a birders experience. Bearing in mind local birds that you might be more familiar with, are the birds you are seeing similar in 'body' shape on the water to say a Cormorant for example? If they are then you are likely to be looking at a Red-throated Diver.
A Guillemot is a much smaller, dumpier looking bird more akin to a duck ie much less elegant with a shorter, thicker neck.
A Guillemot should also show more white on the body of the bird whilst it is resting on the sea and in addition should show a greater contrast between it's black and white plumage, whereas a Red-throated Diver would appear to have little white on it's body and would probably look greyer overall apart from extensive white on the neck.
If you want a bit more info send me an email and I will send you a few pics of winter birds I have taken in the last 5 years or so, failing that get yourself down to Filey Brigg some 30 miles down the coast and there are bound to be some birders around to give you even better id!

Good day today - I spotted several Guillemots and one RT diver - very clear based upon your hints on body shape and plumage.

Many thanks David, and a very Happy New Year to you and to all of the contributors to this forum.

Many thanks for your further note David.

The "mystery bird" in France remains a mystery.  I realised that I was "pushing my luck" in asking for help with this one.  The problem was that the little beggars were very furtive, so I struggled to see them clearly, and hence could not give a very good derscription - I appreciate that, with the rather vague description I was able to give, the chances of there being enough for an identification were rather small.

I did recently find a very poor photo (attached - taken with my iPad, through a window) which might shed a tiny bit more light on this.

If you, or any other forum contributor, can come up with any ideas on this, I'd be very grateful.