Hawkeye,
You can get smaller sleepers that are easier to put in car, carry across site and cheaper (£13 per linear meter):
https://www.wickes.co.uk/search?text=sleepers
or even 100x100 fence posts at £7 per linear meter:
https://www.wickes.co.uk/search?text=fence+post
I would also get some metal plates to screw them together.
SFK
You can get smaller sleepers that are easier to put in car, carry across site and cheaper (£13 per linear meter):
https://www.wickes.co.uk/search?text=sleepers
or even 100x100 fence posts at £7 per linear meter:
https://www.wickes.co.uk/search?text=fence+post
I would also get some metal plates to screw them together.
SFK