Out over xmas so far do in ok in a N/W exposed positions but with light overhead cover of rock ,time will tell !!!

This may or may not be relevant, but I had a clump of what came to me as T. bergeri (a very similar and fairly closely related species), which I split and 'planted' in a couple of parts of the garden. The largest portion was carefully shoe-horned into a cosy niche in a south facing stone wall bordering the garden. I felt sure it would be fine there with protection from cold rains and most likely to survive. The other - a single rosette was wired to the trunk of Yucca elephantipes immediately beneath the leaves in a similar south facing aspect. It was more exposed to cold winds and with a greater chance of getting soaked as winter rains trickled down through the Yucca's leaf clusters. In theory, a bigger risk.

The plant on the stone wall seemed to do well for much of the winter, but by by late spring it was obvious that all was not well. Close inspection revealed that the bases of the rosettes had rotted and by mid-summer the entire cluster died away. Conversely the smaller cluster attached to the Yucca trunk came through unscathed and grew away quite well that summer. The following year it produced its first flower spikes and has continued to do so ever since.

My conclusion was that although blocks of stone may afford immediate protection for a short duration, they become very much colder than either live or dead wood after a day or so of cold weather and this can result in damage even if the rosettes are protected from winter rains. I tried bits of T. juncea in similar spots, but found they could not survive contact with cold stone over a winter long period, although the species does OK and flowers regularly if mounted onto a tree.

This is the clump of bergeri about 2 years ago, some 3 years after mounting it on the Yucca. After nearly doubling in size the following year it was split and the offset is now doing just as well on the trunk of an evergreen Himalayan Cornus with an east by north-east aspect.

It wouldn't have occurred to me that stone temps could make a difference in growing Tillandsias there in England, but Dave's explanation does make perfect sense under sustained cool/cold conditions. Here locally, growing tender plants around/in large boulders will generally give a heat boost, and reradiate heat again at night and keep plants a bit warmer. Then again, we seldom get severe cold for days at a time, and most days do warm up at least 10 to 15 degrees over the lows at night, which makes a difference. Certainly the protection from direct rains, and a south orientation to warm up during the day would help, but maybe insulating the plants from direct contact with the cold stone using dried moss would make a difference, so there wasn't direct contact with chilled rock. The principle is the same as cold stone floors inside a house, which feel delightfully cool in the tropics, but too cold to be comfortable in winter in less balmy climates.