I am looking at purchasing a pram (Steelcraft Strider Mini) which says it’s suitable for newborns especially with a “comfort insert”, but I notice the baby would not be lying completely flat in the pram - their hips would be flexed slightly. Is there any guidance on how safe this is, and how long I could safely leave them in there if eg going to a cafe etc? As I’m not sure whether to purchase the bassinet attachment too. Thank you

Red Nose recommendations are related to safe sleep, but of course babies do go to sleep when out & about in a pram.
As a pram is a transporting device, Red Nose would not recommend using a pram as a regular/permanent sleep environment, particularly if the base is not flat. Always supervise a baby when sleeping in a pram.
https://rednose.org.au/article/prams-and-strollers
While prams do have Mandatory Safety Standards, it is in relation to them being a transport vehicle not a sleep environment.
Babies should always be place on their back on a firm, flat (not elevated) surface for all sleeps, including daytime naps.
https://www.productsafety.gov.au/products/babies-kids/kids-equipment/prams-strollers

See also information when using a bassinet - there are no Mandatory Safety Standards for bassinets in Australia.
https://rednose.org.au/article/bassinets
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/bassinettes