Many thanks All,
I know that HIIT Z5, 6 & 7 training can / will increase FTP, but on the other hand so it appears, does lots of Tempo/Sweetspot. From an `event specific training' standpoint time at Tempo would seem more appropriate. Outside of racing and the need to be able to get some separation from others, I don't know whether VO2max plays much of a role in this sort of event. I ask as I am open to either/both and interested to learn. For sure I'd like to be able to channel my inner Jens Vogt, but right now it's more a case of exorcizing Homer Simpson.
I'm hoping that ~10 hours a week of Z2 and with some Z3 added in will push up the LT1 and also a lesser extent LT2/FTP. I would anticipate riding this sort of event at the upper end of Z2 with some excursions towards FTP as necessary here and there. I suppose the question is; within the available limits of time and recovery, whether more time at Tempo or efforts in Zone 4+ helps more.