Help identifying a house visitor

Hi All

Yesterday in north London it was hot, and we had the back door open to get some air in.  We also got a small bird in too!  Have tried googling to identify it but no success.  With all the excitement we didn't think to get a picture, but was hoping experts here might be able to help.

It was a small brown bird, a bit smaller than a robin.  Two distinctive features were its quite bright orange orange triangular beak (finch-like) and two thin white chevrons on its tail feathers.  The tail feathers were pretty short.  Its eyes were teeny tiny.  Overall it had a bit of a 'podgy' look, not as rounded as a robin, but not as streamlined as a bluetit.  It didn't make a sound for the fulll five minutes it was in the back room, so no clues there.  I must say, it seemed very chilled out and was more interested in checking us out than getting back outside.

Well, I hope that's enough to go on - any insights very welcomed.

Many thanks