Relax, this is not a false widow, it is a harmless missing sector orb weaver (Zygiella x-notata most likely), a very common spider around & sometimes in our homes. Yes there are likely to be more, I should imagine every home in the country has some, I have half a dozen inside my living room window frame, countless more outside. They particularly like window frames, so if you help her find her way there she'll prbably stay there (inside or out).
They're quite fascinting really, their webs are often missing 2 sectors (hence the name), they hide out in a little web sac during the day at the cornerof the web, then hang in the middle in the evening/night. Quite cute once you get used to them - I'm a recovering arachnophobe myself ;-)