If Maplins are NFG (and godnose they so often are…) . . .
If you can get the knurled collar undone and free the tip from the iron, you should have some uncorroded metal at the base of the tip to measure the diameter of shaft and flange - that will give you a guide to what will fit from other manufacturers. Then it’s a matter of looking for that size on ebay, or finding data sheets with sizing info on.
Alternatively, most Maplins stuff is generic Chinese which is marketed elsewhere under dozens of names - so you can search ebay and Ali Express for visually similar units and find tips for those which will almost certainly fit, or be cheap enough that it’s no biggy if they don’t.
When you find the right one, buy plenty so they never again end-up looking like that sketchy POS - guaranteed fubar solder joints using anything that looks like that!