I haven't started googling yet as this has just occurred to me, so please humour me - I'll go off and do some research shortly.
Has anyone been mountain biking in Tenerife in January? I've not been there, but have been to Fuerteventura - from the photos I've seen the terrain looks similar. Is it the sort of place where you'll only get something out of it if you're a super brave/gnarly MTB person? Did you take your own bike or did you hire out there?
In reply to Tall Clare: Check out http://www.lavatrax.com/ they do everything from quite technical stuff to "the long way down" which is basically a 50km+ gentle role downhill from Teide national park to the see to xc stuff on the north of the island. They'll provide bikes and all the gear you need too. Darren who runs it is from Yorkshire as well so a good sort.
Went out a couple of years ago just after Christmas and proved that you can fit two hired MTBs in the smallest available hire car. Hired bikes from http://www.bikepointtenerife.com/home (who were very helpful).
We stayed on the south side of the island and although it is basically one huge volcano, there are some relatively flat trails following forest roads (one in particular north of Vilaflors, and along the coast). In the main though, for us it was quite steep but off road we used the forest tracks so they were not too technical. Our last day was to start from near the top, descend 650m and then make our way back up again. Neither of us are super gnarly or brave and are quite happy to push the bikes uphill as needed! If you want to just go down, people put the bikes on the bus, which takes them to over 2,000m and then there is a 75km descent to sea level on the north side of the island. You can find out about that easily by googling.
One thing though - we stayed at about 700m altitude (Granadilla de Abona) and it was cold in the evenings. We did get a bit jealous of some friends who live on the coast and enjoyed the January sunshine...