For the last few weeks I have enoticed that the grass is not flowing back into the catcher properly - it builds up at the entrance of the catcher and then doesn't fill properly.
I have cleaned it to ensure airflow is not blocked.
Appreciate any tips before taking it into the repair centre.

HI,
First question; Is the grass wet or damp? That will stop it before anything else.
Do you have a push mower? Riding mower? When you say it isn't flowing "back" into the cather, is the catcher in the back?
Usually, this is caused by buildup under the blade(s), which hinders free movement of the grass as it's cut. It wants to stick to other stuff over the blade, before it gets a chance to be pushed into the catcher.
I do wish you the best, and maybe some others will have some answers also.

Ditto on the wet or damp grass, pushmower or riding mower, the wet grass will collect above the blades at the top of the deck, more grass sticks, and it cuts the air flow off more and more until it just clogs.

The mower is a standard two stroke with the catcher at the back.
The grass is occasionally a little damp - but this is Sydney Australia and I have been waiting until the late afternoon to cut it so the sun (temps 24 - 35C) has dried it.
If the blade was a bit on the blunt side, would this mean the grass was being smashed rather than cut; slowing down the process and making it more likely to stick?

Yes it would, is the blade dull though? IF it is, get it sharpened or replaced, either one you do, make sure the blade is balanced.

OK - I will book it in for a service - been couple of years probably since it was last in.
Thanks for all the help

Ahh, couple years... if the service is free... I'd go for it, but otherwise its pretty simple to take the blade off, and either get it sharpened, or replaced. Just make sure its balanced! Nail in a post, and hang it by the bolt hole, making sure it evens out level is the basic way to see.

Is sharpening as simple as taking to it with a file or not worth trying to do at home?

Well, its best with a grinder, some shops charge pretty cheap prices to do it, if its not too dull or messed up from rocks, I usually just do it with a file.

Thanks mate, needed an excuse to go and buy an angle grinder

Try a product called MowerGUARD. It will help.