All four cars that are going are couples (Aged between 40-64) and the trip is about aweek long stopping in different hotels along the way, which I am finding is becoming a bit of a night mare because all the different cars have different abilities at soaking up the miles. Which is playing havoc with the lenghts between the stages I would like.

It would be easier if all the cars were Lotuses
The Exige is very good at at covering distance even in the fine Scottish rain. The other car may well be a Porsche which will also not have a problem soaking up the miles as well. But The two Caterhams may pose a problem if the weather is like it was when I last ventured up that way. I am not sure what the pace will end up like, although no one is racing. The Caterhams which are normally quick cars may slow things down a fair bit if it rains.

The windscreen wiper just tend to smear the screen

and it is not fun being in a 7 with the roof up for long distances..
I only say this as a warning because the pace my be slower than you (Or I) would like.
Some parts of the trip have already been booked. Would you be wanting to join up for the whole trip or just for certain parts?
The trip will be finishing at your home town of Edinburgh.
Tickets for the Tattoo might be a problem because I had problem getting them for us all to be together when I ordered them in December. Mind you you might be feed up with the Tattoo seeing its in your home town.
Would it be just you or you & your partner?
I don't see any reason why you couldn't come along, It would just need a bit of sorting out that I hadn't planned for, But that is not the end of the world. Originally I was just planning this for me & my partner which would have been easier because we wouldn't have needed anu real planning. Amazing how things change
Let me know your thoughts.....