Thanks for your input vand, that helps.
I think one question I've been struggling to get clear myself, which your post helped me process some more, is also our need to clarify which route is best for our situation... a) drawdown or b) living off the interest from our ISA and / or SIPP.
Although the SIPP is better financially, if we decided to save to that from now, we'd be pushing ourselves down the drawdown path for the ISA if I did decide to stop work from 50 (as the ISA pot would stay at around £74k plus interest as we now start saving to the SIPP). This would be enough for 7 years onwards as you said, but then would run out, though we would then move onto the SIPP.
If we managed to get to £120k in the ISA by the time I'm 50, we could live off the interest and not touch the capital (possibly with a little work to top up our living costs).
As I mentioned, we don't want to end up with too big a pot in the very long run but I'm wondering now that I'm untangling it, if the benefit of continuing with the ISA is that it leaves our options open with regards to drawdown and living off the interest... Especially considering not being sure about how long I'll work, hours I'll get etc.
Does that make sense? :)