Update! Yes, it’s taken almost three weeks for me to resolve this issue. This is the sequence of my attempted resolutions:
1. Contacted plumber who couldn’t come out for more than a week
2. Decided to hire a wet vac, only one I could get locally was £58 a day.
3. Called another plumber who said he’d come in a couple of days.
4. A friend came round and between us we decided to try the One Shot.
5. It worked! Lots of gulping - a good sound.
6. Had one stress free shower then went away for the weekend.
7. Come the Monday morning the drain had stopped clearing again.
8. Poured more One Shot down.
9. Several calls to plumbers and drain people (interesting to note one plumber wouldn’t come out because he said all he would do would put One Shot down there anyway).
10. Finally spoke to a helpful drains guy who helped me to ascertain that it was a pump issue and not a drains issue; he told me I could get a service kit for the pump for about £17.
11. Decided to call the pump manufucter’s (Whale) maintenance line.
12. They suggested there probably was a blockage that One Shot cleared but then One Shot may have caused damage to the pump valve.
13. They posted a valve to me (arrived today, one week later) along with a link to maintenance videos
https://www.youtube.com/user/WhaleHealthcare
14. I fitted the new valve; the old one had indeed hardened explaining the outflow issue.
15. It worked. An emotional moment.
And the moral of the story? It’s all a learning experience for which I am very grateful!