Either read on or digest it all in a 1 minute video :)
After some brainstorming, the plan became to make some strategic cuts with the angle grinder to seperate the top half of the frame triangles away from the bottom half.
One potential problem with this approach would be the strain put onto the join that used to be the top of the seat tube. We solved this by welding a smaller diameter section of steel pipe inside and capping both ends with metal plate.
Once welded up we found out for the first time whether it would actually work. The frame had alot of bounce to it, but that's good as long as it's not too much. We knew that once the board supports and the plywood footboard were fitted they would add a bit more rigidity and the board itself would take flex somewhat in favour of the frame.
I had a can of purple spraypaint sitting around with no purpose (only bought it because it was £2 clearance). Even with the frame having virtually no preparation, the paint finish turned out really nice.
The finished scooter is alot of fun to mess about on. It's never going to be used as a proper dog sled, our dogs are neither big enough or trained for that, but it's certainly more use & fun than a broken Asda BMX :-)