Bit of drama here: the shower which had been taking a little longer to drain but had been manageable suddenly... wasn't yesterday, so I had to find a plumber.
To cut a long story short, actually found one via the local FB group who said they could come today, and was actually able to come earlyish this morning - sorted things out in 15 minutes including putting the tap in the shower back on and only charged me the callout fee. And thanked me for not using Drano.