Hi Lizzie,
I've been with JY to various countries in Africa and SE Asia where mossies are prevalent and to the best of my knowledge, no one on any of my tours has ever taken a mosquito net! Most places have air con - keep this on whenever you are in the room as they don't like it and are unlikely to bite when it is on. Use repellent morning and evening liberally - most people use DEET as it is the most effective (50%); however it is quite hard on the skin and the fumes aren't pleasant. Thanks to another JY travel buddy, I've discovered that Boots now do a "light fragrance" mossie repellent which is now recommended for tropical climates and has a strength rating of 3 out of 4. However, do be aware that it is hard to avoid bites completely - some people say wear long sleeves etc. which is not really feasible when it is very hot and, to be honest, they can get through clothes if they want to - I was bitten under my arm and the middle of my back recently in Crete - and another traveller got bitten on the bum!!
The good news is that most places visited by JY are low risk for malaria, so the worst that can usually happen is you get a few annoying bites. I did take malarone (anti-malaria) in South Africa and Zambia (the only place I've ever been where there have been a lot of mossies) as well as in Namibia (from Damaraland onwards). Although Etosha is classified as being at a higher risk, I only saw a couple of mossies there and it wasn't a problem. This is a fantastic tour, one of my favourites, so have a fantastic time!
Cheers,
Bob