What's this book? Sisters not allowed modern things

[outedbyspoons]

In the late 80s or 90s I read a book in which:

• There were two (I think) sisters who weren't allowed to wear modern clothes, have TV, or even go beyond the garden gate without an adult.
• The sisters lived with their grandmother for some reason.
• There was an archaeological dig in the garden or an adjoining field that belonged to the house.
• An American cousin came to stay, who wore short skirts and other modern clothes.
• The American cousin took the sisters to the cinema (cue amusing culture clash dialogue in which the sisters don't understand why the leading lady's clothes change between scenes and keep mistaking her for a new character) and bought a television that was smuggled into the house.
• A gold cup is found during the dig and the American cousin steals it.

Any ideas what this book ls called or who wrote it?

Sounds really good!

I think I've read this. About to fall asleep so placemarking - I'll come back to you tomorrow (well, today!) because I'm sure I should be able to find it if it's the same one! Was there any kind of incident during said dig? Was there an older woman (maybe archaeologist type, but not necessarily) involved with it? Can you remember the cover and did it feature mud?

I've a vague memory of a girl called Tatty or Tattiana. Not sure on that, but I'll do a bit of digging and come back tomorrow.

The older woman I recall was the girls' carer.

I don't remember the names of any characters.

I have tried lots of versions of this on Chat GPT but haven't got an answer that makes sense, maybe you could try since you remember it better?