Wagtail

Grey wagtail. Re doors, it will be responding to its reflection, so cutting that out should stop it wasting precious energy and risking injury. They over Winter in UK so might be happy where it is, reflection aside.

Thanks for the replies. I thought it might have been the reflection, but nothing I can do really. Size of the doors total is 3mts x 1.8, so lots of glass. Will try closing the blinds a bit to deter.
Are they common in central Scotland this time of year? Never seen one in the garden before and had the pond for some time now. I have feeders out which we get quite a few varied visitor most days, Blue Tits, Great Tits, Crested Tits, Sparrows, Wrens, Wood Pidgeon's, Starlings, Magpies, Jackdaws, Blackbirds, Collared Doves and the odd visit from a Kestrel and of course, the pesky squirrel that teases my dog who tries to knock the bifold doors out to get to it :)
Had a hatching of Blue Tits last year and 2 Wood Pidgeon chicks.

It'll just be the area the bird will be stood or perched, so no need to look at the total surface area. Blinds often do sort the problem, but a plant pot or potted shrub may work too.....

I can't comment on how common they are where you are. They tend to be solitary at this time of year, so I wouldn't call them common anyway. They do spread out from breeding areas, and I get one here for short periods some Winters (SW England).

You get crested tits in your garden? Approx which bit of Central Scotland is that?

Map 

Falkirk-ish according to Bing

Get the kettle on, I'll be round in about 10 hours lol

Hi, I am in Larbert, right in the middle of Glasgow & Edinburgh, near Falkirk & Stirling.

Thanks. Won't be crested tits there. Coal tits might be what you've got? Crested tits are 'central' in terms of the middle of Scotland, Cairngorms, in coniferous forests.

Aye, we have coal tits too. Do they have crests/Mohicans too?

When the wind blows they do lol

If you have a photo of the birds in question we shall be happy to take a look ;-)