Mine came in handy when I destroyed a chainring in the 24 last year. @scherrit had the original cranks back on the shiv in a lap of Quina Brook on my px :)
I still have my PX from last year, with my schedule (519) sellotaped to the bars... But I've lent it to a local chap who's new to TT but was then undiplomatic enough to beat me.
I think plan will be to fix the Shiv with some new bars for training plus race but definitely take two bikes.
You never know what else might fail and when you're at the pointy end it matters. Especially in your case going for a comp record you want that safety net.
Do you have a spare bike?
Mine came in handy when I destroyed a chainring in the 24 last year. @scherrit had the original cranks back on the shiv in a lap of Quina Brook on my px :)