Could be a variety of issues from poor quality parts to attack from various bathroom products. Standard replacement parts readily available and certainly diyable. Have a flick through hosts catalogue and ask questions if you need any help....
Just a pair of these or similar https://www.screwfix.com/p/irwin-vice-grip-12-groove-joint-pliers/43518 To undo the nut. I always use silicon sealant to seal the new parts too, but others will disagree on this
Another one who like to use a sealant on the parts, 'Plumbers Gold' is what I use. Alway like to bed basin,bath sink outlets on a bead of it, despite the sealing washers.
Thanks everyone with you fine I think this might be the culprit although I only used it twice in the four years the sink was installed. Bleach would be the only other liquid used with frequency.
Great stuff when used on materials suitable,cant say more as the bleach companies have deeper pockets than me. Dilute fairy liquid (As an example) is the best wash down approach,if its harsh to handle then its also harsh to your plumbing pipework.
I successfully replaced the waste today (plumber's mait) - took me approx. 5 hours (I know). Look at the extent of the corrosion.
Or charge hourly ! local company here charges Standard Rates (per hour) Rates Mon–Fri 7am–7pm Plumbing £100.00 + material + vat.
Yeah. Cheap materials. Need brass base. That looks like a cast alloy, copper coated and chromed. By the way, NEVER use Harpic on Chrome! Mr. HandyAndy - Really