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Thanks pastafreak. I understand what you say about the collar getting caught but it has an elasticated bit that, so far, has been absolutely fine. Only thing that has happened just 2 or 3 times, she can get a front leg caught in it if it's a little too loose.
My other half uses Frontline on his 7 cats, which seems to work for them, & it's easily available, only reason I mentioned that one in particular.
This has only happened since we moved to a more rural spot, so think she's either picked it up from wildlife (she's become a mouser too :( ) or possibly from the carpet as there were 2 dogs here (& the previous occupiers weren't particularly clean, euch) but then we've been here 5 months & this is something new.
Definitely like the idea of getting a spray so will look into that :)