<Dredges back through memory> The following may not be entirely reliable
There's probably no more than a mile or so of tarmac though some of the other tracks are pretty well packed - in addition to the deep muddy stuff through the woods, there will also be plenty of wet grass and bare earth. I'd probably go for grip over 'road comfort' if I had a choice, particularly given the weather forecast, but then I'd also put both pairs in the car and make a decision on Saturday when I've seen how much water is on the racecourse. Your avatar piccie is taken at the Hawkshead Trail isn't it? Whatever you wore there would probably see you round - I say 'probably' because I have seen folks doing Hawkshead in road shoes which are definitely NOT suitable.
If it makes any difference, I'll be in my older pair of Inov-8 Roclites which I find very good on all terrain. I'm keeping the newer pair back as I break them in for Hawkshead next month.
I'll look out for the Bourneville vest - and try no to confuse it with the Helm Hill Runners vest worn by the club who train on the hill over the road from me.