First time I've grown this variety but have noticed, with the second trusses now appearing on all three plants, only a maximum of three out of six or seven fruits have developed from the flowers.
Is this a characteristic of this variety? The tomatoes look as if they're going to be rather large, and despite a weekly feed I'm wondering whether this is the plant's natural way of preserving its energy.

Last year mine all set, but this year the ones flowering during the hot spell a few weeks ago have only set a few per truss. You may get more setting now the weather has cooled down a bit. More likely the weather than the variety.

This is a characteristic of a lot of beefsteak varieties: they don't set well during a hot spell. You'll find they'll set much better now that it's cooled off.

I can post you a rabbit's tail to help with pollination. By the way, the rabbit was delicious. Cheers, Tony.