Apparently ONN/Durabrand products from Asda are often Vestel clones (rebadged) products, along with some examples from Digihome and Wharfedale.
click here (Vestel website, but needs translation uyse)
This doesn't necessarily mean that your microwave comes from Vestel, but if you can find another marque's similar product you could obtain support from the website.
Also check Asda's customer service for the supply of a manual.
A Vestel parts supplier: