So it's been a bit of a long process, and I'm not quite at the holding my breath stage, but we do now have an au pair who is due to start in a fortnight. I think actually, whilst we did reduce the hours and up the money, the main thing we did was 'compromise' on length of stay, and the new au pair will just be with us until July rather than the minimun of 6 months I was searching for.
However, since the new au pair accepting the job, I've changed our search criteria to look for someone who can start in August-Sep and have had about 10 applications in 24 hours (which I know isn't a lot compared to some families but is more than we've had in about 6 weeks with exactly the same terms, just looking for a March start). In fact, the job is a whole lot easier from Sep as the youngest is in school so it becomes far more of a standard au pair job (just with four kids) BUT the prospective au pairs don't actually know that as I've not changed the details as I didn't want to confuse the 'new' au pair who is yet to arrive.
ANYWAY - this was really just to say if anyone else is struggling to recruit at the moment, it seems to be by far a problem of recruiting in Feb/March than anything about Brexit etc...