For anyone interested, in the end I discarded the idea of sleepers. This was due to wanting long lengths rather than lots of short sleepers and a certain height, so the sleepers I was looking at were 2.8 metres long and 25 cm high. They said they weighed 70KG each and with lockdown, only I was working on this, so would have struggled getting them round to the back garden and in place etc. I decided to use long lengths of building joists, which are much lighter and manageable. I also used fence posts to secure the sides after buying an auger drill bit on amazon which worked brilliantly at digging the post holes. The sides are all in now and just need to get the base laid and then get the greenhouse at some point!!