Ricky,
I asked this question many times and the answer I kept getting back was to buy the flushing kit so I did, and it was well worthwhile.
When you get one you have to cap the water production hose when flushing and I found the stop cap you remove from the merlin to fit the flushing kit is a great fit.
Might seem a bit confusing but you'll get what I mean when its all in front of you
Alistair