MM wrote:

Yesterday I bought a FENCEMATE® Swift Clamp Concrete-In Post Support
100 x 100 epoxy brown, but there were no instructions with it.

Page 8 of the brochure [from my local woodyard] has some brief instructions

https://coltmanbros.co.uk/wp-content/uploads/2016/12/Superior_Fencing_Accessories-1.pdf