It's been said many times before but exercise whilst VLCD-ing can stall weight loss. If you are SS-ing this is particularly true. The body holds on to every calorie it can to keep you from 'starving' because it has no idea you are eating around 500 calories a day because you choose to!
If you are on week 3 or 4 of Cambridge you can pretty much expect a small loss or a STS. Again, this is the body adjusting because you probably lost a lot, very quickly, during the first couple of weeks.
The promise offered by most VLCDs is an average loss of one stone per month. It might be helpful to you to view losses over that time,and not just week by week. Don't lose heart! Just keep going.