It will be fun to see what kind of tomato comes from this little one. The most important thing is that you plant it in your gardens sunniest and hottest location. If you decide to plant it in a pot use a large one (since you don’t know what variety you have) with good well draining potting soil. Keep it well watered and as it grows taller provide the plant with a stake to support it. This bit of advice comes from watching my Sicilian grandfather plant hundreds of these plants.....as for myself, I have yet to grow one.

keep it sheltered but in full sun, I'd bring it in at night and pop it out during the day. Did the patient say if it was an outside one? A sunny corner should be ok where you are, but if frosts forecast cover it/fleece/plastic sheeting over night.
As you dont know which variety then I would remove any side shoots that form in the leaf axils [between a leaf and a stem]. When roots are showing through the bottom of the pot repot in a quality compost. A pot like ones roses come in or slightly wider would be ideal. [I'll post a photo of mine that have just gone in their final pots later today] Add a bamboo cane to support it as it grows. When you have 3-5 clusters of flowers then start feeding it with something like tomorite.

I forgot to go back out to the greenhouse. will do so in the morning Kate.

Many, many thanks to you both, Loosestrife and Eileen! Apologies for my lateness - works been hectic!!!
He had it in a cold frame and not his greenhouse. I do appreciate all the fantastic info and guidance - I shall endeavour to follow.
Ps. Loosestrife - you must try to grow them, especially as you have Sicilian blood!!!

I've written a short blog for you Kate, with the photos I promised.

Thank you, Eileen!!! 😉