Mixer bath tap has been leaking (hot tap, not that which one it is matters 
)
I've been to two large DIY shops, scoured online and cannot find a replacement. We used one that is different in design (flat), but it's dislodged and I'd now blocking water from coming out of the tap in any quantity.
I've attached some pics to show what it looks like (two photos and one really poor cross section I've drawn). Does anyone know what I should be searching for? It's 3/4" rubber, seems to almost match the delta washers, but not quite.
Help!