Good start, Bill. It is quite close to King's Cross but not Pancras Road - go south.

The optical illusion was what I was aiming for when I took the pic. Might it be called Escheresque?

Yes, Escheresque indeed, quite hard on the brain! Would really like to know where this one is!

There will be a clue. Not quite time yet...

Quick clue (before everyone starts to feel dizzy!) Arts and crafts...

A good arts and crafts guess, Bill but it's not the Euston Rd fire station - you still need to go south...

Mary Ward Centre?

Yay! Youve got it. Well-spotted! Mary Ward Centre it is.

Actually, I think it's the Mary Ward House in Tavistock Place, rather than the Mary Ward Centre in Queen's Square.

Yes, sorry, MW House. I was imagining that was what you meant. I know the difference between the two but somehow muddle them up when talking about them. Duh!

I can't spot this window anywhere on StreetView, however.

Ooops! Was convinced it was MW House as I took several pics of that building that day. However, now see from street view that this arch is actually the doorway of the building a few doors down.
goo.gl/maps/uXp6q
(Oh dear, must send myself to naughty corner.)

I'll forfeit my point ;)

Or maybe you should have an extra one for being so vigilant and spotting my error ;-)

I looked at Mary Ward House on Street View also. I guess neither of us went far enough down Tavistock Place.

I'd be rubbish at GWL if not for Street View, especially as I spend only 9 or 10 days a year in London. Thank goodness using SV isn't cheating.

(Hangs head in shame.)
Sorry for all the misleading information that has led everyone on a merry dance. What more can I say? Please let me know what you consider to be a suitable penalty! In the meantime, maybe I should award both of you a point? The alternative would be to send it to the graveyard but we don't have one here at the moment and it's not easy to administer such things without sticky posts. (On many people's ipernity wish lists.)
I took this pic when I was walking along Tavistock Place on the way back to work from a meeting. I was randomly taking photos of bits and pieces that I thought were interesting and I also took some of various details of MW House. When I downloaded the pics I was convinced that this was also that building and didn't check sv until MJ pointed it out. Once again, apologies.
Btw, I am totally convinced of the locations of other pics I have in the pool at the mo and won't be posting any more for a couple of weeks as I'm about to go off to Germany to visit a friend.