Thought I had better sort out the seat attachment/security issue today. I decided I will not weld back on the lock and refit the cable rather go for a latch under the seat opened by a 4mm allen key. Obviously not as secure as the lock but only I (and readers here) know how it is done.
3 steps to achieving
outcome 1. make a seat latch plate, 2. have a level place to bolt said latch plate and 3. make a lever mechanism operated from below that actuates said latch.