I predict you're going to get a lot of varied answers to that, Simon, although my crystal ball does show that the majority are likely to say start now :-)

One particular thing to pick out from your comments:

My idea is the potential lower capital gains from the HYP are offset by over-weighting the smaller companies in the 250

There are a couple of flaws in that statement. First of all, while there may be "potential" lower capital gains from the HYP (and some here will say otherwise), while you are in the pre-retirement phase, and don't need the income, what you are interested in is total return, which comes from capital gains plus reinvested dividends. The total return you get from FTSE 100 HYP companies should be no different from that you get from FTSE 100 non-HYP companie.

Secondly, you may well get greater returns by over-weighting smaller companies in the 250, but that'll be because you are taking greater risk. It's not free. Of course, there are some broad generalisations in those statements, but they are, nevertheless, fairly axiomatic.