I Dont Know What Bird I Saw

[malagabob]

I spotted and watched a bird I could not identify at the bottom of my garden.
It was larger than a blackbird mottled brownish on its back and front wings were black, tail feathers black, bill quite long and black. It may have been a youngster. It was not a thrush. It did remind me of a young gull but it was the wrong colour I have a pretty good knowledge of British birds, but could not identify this one.
Any twitchers give any ideas?

[Chipchopper]

My guess would be a juvenile blackbird, I was watching one in my garden only minutes earlier.

[Gizmonster]

Try this:

https://www.rspb.org.uk/wildlife/birdidentifier/

[Hypognosis]

Nutcracker?
Dark chocolate brown, boldly speckled with white flecks; blackish wings and tail; -conspicuous- white under tail-coverts and white corners to the tail tip. Long, heavy, blackish bill
Occasional/very rare in UK, Spain, France, Neths, Italy (but my bird guide is an old (60s) print.

However, I suspect you meant "brown, mottled with another brown"

Gull is quite a distictive body shape and you've eliminated the thrush shape. I've offered you a corvid-shaped suggestion: are there any other types you can think of which might be a better fit?