Thanks ProDave,
Reasons/logic,
The self occupy a 'caravan' on your own land without a site licence was new information to me, so just planning permission :-). Too much info on the net about within the curtileage of a house/ancilliary use, but I was unable to find info about what I would like to achieve.
A long thin site might mean cheaper land that was unnatractive to others.
Concrete base for a mobile home can be an easy uninsulated slab.
The chassis mentioned is an assembly kit easing transport.
SIPS panels can be 2 person or small machine placeable based on size.
We have recently stayed in an ABI Stargazer 33 x 14 2 bedroom with velux windows, bath in bedroom, washing machine, dishwasher, american fridge, real bed, household quality sofa etc - new price for equivalent from ABI approx £110K but self build should be cheaper as pricing for these things is 'opaque'
Graham