Is it safe to use a sheepskin underneath the fitted sheet in the cot after baby is 12months old? It does make surface softer and uneven. To improve warmth in a big draughty house where temperature fluctuates wildly overnight. Thank you.

Red Nose recommends withholding any soft items (pillows/bumpers/sheepskin) from sleep environment for babies.
However, it is generally recommended not to introduce soft bedding until child is out of the cot or is 2 years of age..
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows

Sheepskin, lambswool can lead to overheating in some cases.
Red Nose recommends adding or removing bedding for baby as the weather changes.
Using a sleeping bag (warmer one for winter) & add clothing, bedding as temperature drops & nights become cooler,  is recommended.
Have a look at Red Nose bedding information.
https://rednose.org.au/article/how-much-bedding-does-baby-need
https://rednose.org.au/article/how-to-dress-baby-for-sleep

Often older children do not keep bedding over them when sleeping, so adding another layer of clothing to baby is preferable.
While this information is written specifically to reduce the risk of SUDI/SIDS many of the guidelines are still appropriate for a child older than 12 months.