It's a Willis immersion heater connected to the dhw cold feed and the hot water draw off.
probably cheaper and quicker getting these and cutting them down a bit with decent snips,...
Send shower doc the pics you have now maybe they can help you now,
maybe here too,...
Work back from the open ends cutting as you go, you wont get confused then.
Should be a metal tank with solid fuel.
Have you connected the screwed rod in one of the water ports?.
Give it a wire brush and take a clearer pic, it looks like an early 70's stelrad triangular bleeder,
Roots maybe entering the drain through a joint or a crack in the pipe.
2 1/2" x 1"...
Do you still have some 110mm soil pipe left over?, use a bit of that and two 110mm 45 degree bends and slip the 110mm pipe
over the 3" steel pipe...
Cut the cast iron back to the wall,
Cut one out from an old leather belt, tap washers were made from leather long before fibre and rubber were used and will last a lot longer.
Looks like a 3" ( 82mm ) Osma soil bend, post a pic of the upstairs vent pipe as it is now.
Separate names with a comma.