At just a quick glance I can see immediately that there several obvious things. There was a wire connected to terminal 1 at the Potterton terminal 1 which is for 'hot water off' yet there is no connection to the 'hot water off' terminal at the Drayton. The switching wire at the Potterton terminal 4 for 'Central Heating on' was a brown wire, but a yellow wire is connected to this function at the Drayton. If you are blowing fuses then you will have joined a live and a neutral together somewhere, but not easy to tell where without spending ages trying to work out what each wire actually does, which is difficult when what the other end of each wire is connected to is unknown. Unfortunately, the wire colours don't help much, as they would have been chosen based on the personal preferences of the original installer.
The main terminals L N, 1, 2, 3 & 4 at the Potterton (there will be diagram on the back of the Potterton showing what each does) correspond to the same terminals on the Two Channel Drayton Wiser. The live loop to terminal 5 can be disregarded as the Drayton has this link internally, and the other terminals are just that. Convenient wiring terminals like a junction box. They don't have any function associated with them.
In this instance, from what I see at just a quick glance, I think that you are out of your depth, and are not understanding what you are doing, and what each wire actually does. You should really call in a professional in to install the Drayton for you. There is a possibility that you may have damaged something already. But it could have been worse!
Thanks for any help. Bit of a novice at this.
