Hi Altonx, do you have the maintenance manual? If not, it is available for free download here
https://www.winget.co.uk/wp-content/uploads/2020/12/PETTER-A-RANGE-WORKSHOP-MANUAL.pdfThe manual recommends heating the glow plugs for 20 seconds. If you do try starting on just one cylinder with the other decompressed please check that the decompressors are correctly adjusted or you may crash the piston into a valve.
How old is the battery? Perhaps it does not have enough voltage or cranking amps for this application. Check the electrical system, battery terminals, solenoid connections and any earthing wires. Check the glow plugs are actually getting hot.
Let us know how you get on.
Bob