Thanks all!
They're ten-year old sisters and they came from Furry Friends in Coulsdon. Sadly their owner passed away several weeks ago, since when they've been on their own with a neighbour popping in to feed them. They were briefly rehomed in a household with small kids but found it too traumatic, and were due to be PTS this week if they hadn't been rescued - so a big thank you to Emma at Furry Friends for saving their lives, and an equally big thanks to JackSpratt on here for putting us all in touch with each other.
The girls are both very affectionate and sweet, and love bumping heads and rubbing against us. Drum is a bit more dominant and very nosey! Unfortunately, she has an occasional tendancy to spit and raise her paw without warning! Bass is a sweet thing, she was a bit shy at first and spent the first night hiding (she also held things up when we went to collect them by hiding inside - yes, inside! - an armchair). But she comes out straight away now and is even happy to sit on a lap.
Bass is possibly the blackest cat I've ever seen - light just disappears around her! Although you can't see it from this photo (I will post better ones soon), Drum is beautifully marked and could be part Bengal.
Obviously it's still very early days but they seem to be settling in well - both are eating and using the litter tray (one of them tried to destroy it last night!) and love being fussed.
Ella and Effie: I think you must have been imagining it, these are the first pics I've posted!