Okay so I wasn't going to post until I'd had some kind of resolution. However, planning called today to ask if they could access my garden on Friday as they need to do some "further investigation" and have another planning official who will be there to oversee it??
They wouldn't tell me over the phone what it was about (it wasn't Nice Planning Lady, it was someone I hadn't spoken with before)
Thanks pp for the thread title - which coincidentally was very apt!
No sign of CFN/ builders etc.
There's now a sign up saying
NO ENTRY IN TO PRIVATE GARDEN
CONTACT SOLICITORS NAME if you require further information.