Help! Ride-on Belt Mower Advice

I've recently purchased a second hand Countax K1850 ride on mower.  This is my first ever mower and so I'm still getting to grips with it.

Having tried to tackle some weeds I managed to smoke the belt so I ordered a replacement.  I didn't go for a Countax belt as they're twice the price so I bought one of these Dunlop Hex equivalent belts.  I checked the Countax parts site and got the correct serial number and specs and ordered a new belt.

Anyway long story short I couldn't for the life of me get the belt on to the cutting deck as it seemed to come up a good few inches short even with the tensioner released.  Having double, triple and a hundred times checked that I have ordered the correct belt and that the belt path was correct I cannot get this belt on.  I was going to send it back but other than order a Countax (which I don't want to do) it seems that these alternatives seem to be all the same and I doubt that others have this issue as I'd find more references to it.

My question to those that may know is, is there a knack to putting a belt on these cutting decks?  The one that I removed wasn't this tight but I assume there has been an element of slacking from plenty of usage but this new belt is way too short.  I did notice that it didn't look like it went in as far into the pulleys but other than that I can't figure it out.

If anyone has experience of these belts I'd love to know your thoughts before I attempt to return it.

Thanks