Can anyone give me any pointers as to increasing tricep mass? They are pretty small but well defined, im using dumbells on them so i could do with a few other ideas for those.

Why are you only using dumbells? Limited equipment?
The best exercises to do are compound exercises- Exercises that use a lot of other muscles to help stabilize the movement. By doing exercises like these, you're able to use a heavier weight, which is how you build the muscle.
Overhead extensions are great for building mass, so are triceps pressdowns using a lat tower- Also, close grip bench presses and weighted dips are excellent.
As far as dumbells go, you can do single arm overhead extensions, you can do dumbell skullcrushers, triceps kickbacks- Good exercises, but they tend to isolate the triceps a little more than the compound movements I listed above, which you should get better results with.

What do you currently do to work your triceps? How much weight are you lifting with them, how many sets and reps are you getting in?

Try either close grip bench or skull crushers w/ a barbell.

Yeah im working out at home so i have limited equipment at the moment,at the moment im doing single overhead extensions and and lying tricep extensions, about 2 sets of 12,lifting 8 kilo's, when im doing the overhead extensions i struggle to finish them and the tricep feels pretty sore afterwards.