We have a new palmetto lawn that is going very well thanks to our landscaper being someone who actually cared about soil prep etc.
I'm currently mowing it once a week. Started off using the grass catcher but more recently have stuck in the mulching plug.
I've heard various opinion on the benefits of long term mulch mowing but also have heard that it can increase thatch build-up etc.
Can anyone assist with some advice?

After a little searching, it look like mulching can indeed advance of create a thatching problem but only when the grass is mown infrequently and mulched at an improper height. If you mow regularly(unlike me) it shouldn't be a problem, but I guess its one of things that is best in moderation.

Using the mulcher function of your mower will help you use the clipping you would usually )I assume, but may be wrong) throw away. All plant matter can be broken down and added to compost. It degrades and returns to soil and the process starts over again.

If you alternate catching the grass and mulching it it won't build up as much and will give the stuff you do leave behind to degrade and return to the soil. You need to make sure that the grass is not getting too long to mulch it though. I remember that when we bought our most recent mower, the shop owner said if you mow weekly then you could mulch every time, fortnightly then it would depend on how much it grows in that time and monthly he said I need to mow more often.

Currently our lawn gets done every 2-3 weeks and it depends on the length as to if its mulched or not.

I think if the grass needs to be cut by half its height then its too long in the first place and you might need to rake it up instead of catching (depending on mower itself) If you want to cut more than 30% of the height of the grass off then you should probably catch it.

I do not have a mulching mower and spread out my clippings on the lawn after mowing. (I prefer to mow with the catcher on as it is less messy.) Lawn clippings are like lettuce - 99% water! You will not increase your thatch - I have none to speak off. The more organic matter you can return to your lawn the better. (Fu told me so)
The 'horizontal compost heap' concept works for lawns and garden beds. (Visit a great gardens workshop for further explanation) I spread out the clippings with my foot, water them in so that they don't blow around and they virtually dissapear overnight.