So I cheated and had 2 goes at this. The 1st one ended up as a lemon swiss pyramid and I accidently used moldy flour (so I didn't even get a chance to eat any!).
The filling was lovely and easy - shop bought lemon curd...
Attempt number 2 came out better. I managed to make it a roll shape and I used edible flour (!). The sponge could have done with a few more minutes in the oven as it stuck to the paper when I rolled it up (hence the bumpy outside).
It tasted nice and was really light. The sponge was fat free - just eggs, sugar, lemon rind and flour.