I've taken drastic action today (but no photos, sorry).
Yesterday's little experiment bore no fruit at all, no bubbles, nothing.
I consulted Andrew Whitely's Bread Matters again and his solution for an inactive starter is as follows:
Discard all but 130g of starter (which is all I had anyway)
Add 180g flour
Mix into the old starter.
I emptied out the plastic pot and gave it a clean - no chemicals or detergents and gave the new mixture a bit of a stir. It's very thick indeed, not liquid at all which is very different from the previous consistency. I'm not convinced.
Anyway, it's back on it's shelf and we'll see what happens.
What's happened up to now: