You're a bit quick with the knife, chaps!
Often these blockages are quite soft, and you can use the mains to blow the sludge out. You have a mains supply for the header, so use that. There are two ways
(First the third way, if you have a water vac (
Wickes £35 or so). Get a good seal and suck on the vent pipe - the 22mm one which loop sover the top. Then suck at the tank connector fitting, inside the tank. Let some water in and repeat. Often gets enough movement for the system to fill slowly)
Best way: Turn off f/e tank's stop cock.
undo connection on f/e tank's stop cock on the side that goes to the tank.
Connect a garden hose via a normal, small size garden hose adaptor (1/2" bsp). Then you connect the hose to the feed pipe, which is 22mm so the fitting is hard to get - BES do a Speedfit one which is perfect. Gunge gets pushed up the f/e outlet into the tank, so you can vac or bail it out.
Could also use a flexy tap connector screwed on that stop cock, other end is 15mm. Use plastic plumbing pipe to fit to vent via 15/22mm adaptor - again Speedfit.
Second best way:
Use a (long) 15mm to 1/2" flexy tap cpnnector. Connect one end to that stop cock and the other end to the pipe which is connected to the f/e 's output tank connector. Then when you turn on the stop cock the gunge will come out of the vent which you must have a BUCKET under - you've disconnected the tank's ouput so you don't want it in there. If you have the bits to connect a hose to the vent pipe you can push the hose down the overflow pipe and the grot goes into the neighbour's garden.
(First you had to empty the f/e tank!)
If you don't have a stop cock on the feed to the f/e tank then fit one.
Once you get the system filling, even slowly, you stand a chance with chemicals.
Another way to just get the system full of water is to connect a mains hose to a drain cock and open up. You really need about 3 people , to watch things, though.
You have to be a bit careful putting mains on a heating system - if nothing's moving don't let the pressure build for long!