I will jinx it by posting this but I think we might have found a routine that is working for us...
My mum & DS's nursery weren't convinced he was completely ready for 1 nap so we agreed this compromise:
Wu: 5.30-6 (but getting to be 6 more often than 5.30 now)
9.30- WD & quiet time in cot. If DS falls asleep, give him 20 mins maximum & must be awake & up by 10
Nap: 1pm if he slept at 9.30
12 if he didn't (after an early lunch)
DS usually sleeps 1.5-2hrs
BT 7pm (set, regardless of number of naps or nap length)
Following this routine DS has been doing alternate 1 & 2 nap days, is not getting too tired & the EWs are becoming less frequent. I think EBT was throwing us off & contributing to EW but now the 2 nap days seem to be counteracting the tiredness from 1 nap the day before so we can stick with a 7pm BT everyday.
I'm hoping that he'll start moving to more 1 nap days himself as he's ready for them.
Thanks for your advice & support, hopefully we're sorted now...or at least until the next blip