Ok, for a 'spanner only' ( no soldering needed ) modification.........
You'll want one of these ( compression 22mm x 3/4" centre tee )
One of these ( 22mm x 3/4" bent flexi )
and
One of these ( EV mounting bracket and wrap around clamp ) if you don't already have one.
Cut a short piece of garden hose and connect it to the heating pipework via the drain off cock ( DOC ) next to the PRV pump. Put the hose into the pump and open the DOC to a rate that the pump can handle and use that to evacuate the water from the heating circuit. Open all of the rad air bleed nipples to allow the remainder to drain, starting at the top of the house and work down. When dry, go back around and close all the bleed nipples, ( making sure not to miss one ).
Strip the EV etc out of the attic, and cut / remove / cap the pipework and redundant equipment accordingly.
Cut into the 22mm return pipework under / to the side / or above the boiler where most convenient. Fit the compression tee linked above.
Fit the flexi to the EV, hand tight, and work out where the EV will fit and mount it so the flexi reaches the tee without being stressed. When happy with the dry fit, strip the flexi back off and apply 20 turns of PTFE tape the male thread and tighten it into the tee. The other end of the flexi should have a rubber seal so only needs to go a bit over hand tight. Do not over tighten.
Check the secondary EV for .75 - 1 bar pre charge BEFORE refilling the system.
Fill to 1.5 bar and test for leaks. When satisfied that all is A1 and holding tight, run the heating for an hour to get everything nice and warm and recheck. If happy, re-treat with 2x inhibitor and go open a beer ??.
edited to add : with a house and system this size, it may be a good idea to cut a pair of 22mm gate valves into the flow and return to isolate the heating system from the boiler room. That way, any further maintence can be done without draining the whole system and without losing the inhibitor.