Cannot help with unusual varieties, but can tell you that D. pulcherrimum survived a frost here a few years back, where the temp. inside the greenhouse, under bubble plastic reached -8c. Heaven knows what it was outside, but nearby recorded -25c.
The plant is in well drained sandy silt with a little shelter from a large Pampas grass.

I think I should add that we are a group of amateur gardeners, and we don't personally sell seeds. There is a tab at the top of the page where you can browse through advertisers and mail-order sellers, though. Good luck.

I recently grew Dierama ignea from seed bought from Chiltern Seeds. It flowered last year (its 2nd year) and they were a little redder than D.pulcherrimum. The plant is much smaller, at about 18 inches.

I grew some dierama seeds from plantworld called 'plantworld jewels' mixed colours, all flowered last year in their 2nd year just like yours Volunteer, they've all come through the winter unscathed, plantworld have quite a large selection of dierama seeds to buy.
Sorry forgot to say I'm in Lincolnshire too :-)

Mine is in a sheltered position just under the overhang of another plant, but on the sunny side! That probably helps with the overwintering.

Thanks to everyone that has replied, you’ve all been really helpful. Thanks Julia