Don't blame you Mark!
Did you have any problems fitting that tank such as different size hoses, need to extend hoses or anything?
It looks an easier tank to fabricate a bracket for than the one i've used - half the reason i've not made a bracket for mine is i've been hoping to find a bracket in a scrapyard or find the one like you have, complete with bracket etc.
If you've got rid of the airlock, i think unless you have any other problems it would pay to leave it alone - if it ain't broke don't fix it!
If you do have to flush the heater matrix it would be better to reverse flush the whole system.
Get some flushing agent such as Radflush or Comma Xstream Rad Clean
(i used this in my Volvo and it shifted an immense amount of sh!t!) and add it to the cooling system as per the instructions on the can/bottle.
Normally it says run the car for an hour/day/week or whatever, with the heater controls set to "HOT". I left mine in considerably longer which was a bit risky as some of these products can attack the solder in the rad and/or heater matrix but mine was in dire need of flushing so was a calculated risk. The way i saw it, i needed to replace the heater matrix due to poor flow so leaving a flushing agent in for extra time was "kill or cure" - as it happened, gladly it was cure!
Then drain the coolant system, remove the thermostat and the highest rad hose you can find. Now get your garden hose (still with the heater set to "HOT") and feed it into the system so it goes against the normal direction of flow. For example, put the hose into the top hose so it flows towards the thermostat housing and then watch the filth flow out of the stub on the radiator! If you feel adventurous, disconnect the heater matrix hoses one at a time and use that as your flushing point to insert the garden hose.
When you're all done, i found it was helpful to flush/reverse flush/flush the rad and heater matrix again on their own by disconnecting the hoses from them.
Refit all hoses, the thermostat with a new gasket if necessary and fill with a 50/50 mix of anti-freeze and water. Don't make the mistake i made (a long time ago!) of filling the anti-freeze then adding the water, it takes ages to mix properly within the engine and the concentrated anti-freeze caused problems until it mixed properly (leaks etc) by which time it had all leaked away anyway!
Run it up to temperature with the expansion tank cap on loosely (this helps to eliminate airlocks) but keep an eye on it in case it starts overflowing. Once up to temperature, leave to cool (preferably over night), top up and fit the cap properly.
Keep an eye on the level for the next few days in case some leaks have found their way into the system e.g. a Jubilee clip giving up the ghost (might pay to have some spare new Jubilee clips when you do the job, just in case) or maybe one you've forgotten to tighten fully (we've all done it!
This is a procedure i've developed and followed over the years and seems to work well with almost any car i've had, giving a first try-final fix result each time. The only car in question was my old 420 GSi but the water pump went months after doing the head gasket and strangely didn't leak, just sucked air into the system! Never before (or since!) known of that and sadly because it kept the coolant at the top of the engine (ie round the sensors) while building up a monumental air lock, the bottom end seriously overheated just before the top did, by which time it was too late and the engine was FUBAR!
Hope you find that helpful if/when you decide to flush the system again. Apologies if it's a bit too detailed, i've written it so that someone else viewing the post can use the procedure even if they're fairly new to the mechanical side of things.