Old hot water tank fitting

I've been adding a couple of radiators, and have found that the radiator circuit is 15mm (1/2") right from the boiler, so I'd like to increase to 22mm, but.....
The flow side appears easy, a special coupler appears to have been used on the diverter valve, so I guess it will be easy to get a 22mm fitting, but what sort of fitting wouild it be? it looks like a compression fitting, but the present 15mm pipe lookks as though it has a flange on it.
Harder is the return to the tank, which I think is 1 1/8 inch, it has a circumference of 3 1/2", and this has been reduced to perhaps 22mm (circumference 73 mm) to go into my fairly recent back boiler.
But the way it gets into the tank beats me.
There's a sort of brass T, with the 1 1/8 pipe sweated into one side, the other side has a female thread with a flange, with a piece screwed in with string round the threads, and the other side of this is a 15mm compression fitting that accepts the radiator circuit return.
In the top of this item is a fairly massive tee with one side going into a stub of pipe which comes out of the actual tank fitting which appears to be 1 1/8, and the top has a deliberate 1/2" compression fitting with a pressure relief valve on it.
I'm not too clear how it connects to the first fitting, there appears to be a big flanged nut like a compression fitting, but they appear to have been made for each other. This all started life as a coal fired back boiler system.
Questions -
I am nervous of attacking the tank, but I have to, to end up with a 22mm stub of pipe, then I can chuck the rest away with a couple of 22mm tees
Can I get a tank fitting that will do this - its a cylinder about 16" across and 3ft6" tall?
Anyone know what the other wierd T type fitting is?
Very grateful for any help, I guess really I could fit a new tank, but I'm not going to do that this year, but I would like to use 22mm for my main rad circuit and branch off with reducing T's
mike r
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