Calor Lite contains propane gas while your 7kg cylinders (blue ones) are full of butane. Therefore you need to change the hose connection. If you have a 30mb regulator then this will suit both propane and butane. If your regulator is the older 28 mb type then it needs changing. If your van is post 2004 then the new 30 mb regulator should be fitted as standard.
There is a weight advantage with Calor Lite but it is not necessarily as much as makes a big difference. The weight difference is about 7 kg on full cylinders. If you plan on a lot of cold weather touring then propane is abetter gas to use, and the change may be wortwhile. If you only tour in the summer then the advantage is less obvious.
One other way to lose weight is to use only 1 cylinder instead of 2. Use a Camping Gaz as backup and you will get the same weight gain, without any change.
The BP Lite cylinders in 5 kg form is even lighter than Calor Lite but you will need to pay another deposit, and although it is available at Homebase stores, that may not always be as convenient.
I have looked at it but decided to stay as I was as the advantage was not that much. If Calor sold butane in the Lite format then I would be more interested.