Sink drainer seal/gasket

[elyob]

I took the drainer plumbing apart as it was quite gunker up. It's of the type as below. The seal or gasket that stops it leaking is in pretty poor condition and I need a new one. Is this the sort of thing that is easily gettable? Are they standard sizes? B&Q?

[The Natural Philosopher]

Use silicone. Apply carefully and tighten part way. When set tighten a bit more.

Next time clean with caustic.