Rough running on start

Plugs are ngk iridium, horrifically expensive but last a very long time. There is some variation in smoothness in the m13a that seems to relate to servicing, some I have heard are like tractors and others like mine are barely audible and smooth as satin.

Suzuki recommend both Iridium and Nickel plugs for the M13A, so you don't need to fit the expensive Iridium type just to keep the engine happy. You need to balance the cost of the various plugs with labour/time to change to determine which work out more cost effective plug for your circumstances, or just fit whatever floats your boat.
I have used a few types in my own M13A and never noticed any performance difference, so it is purely a life of plug thing.
These are the plugs I have used, they range in price from £2.50 to £8 ish each, currently on the most expensive the BERU which has an extended service life compared to the others. On the subject of service life you need to look at the individual manufacturers data sheet, they tend to quote a time / max milage to change figure, even Iridium type plugs don't last forever.

BERU UXT14
Bosch Super 4
NGK BKR6E-11

It's another subject that has many options, it's just like the subject of "which oil?" No wrong answers, but in general anything new is usually better than old and worn out.

Have fun
Mike

Thanks for your input Mike - really appreciate your experience (or anyone's) and the recommendations

I'll try and find time to pull the plugs and have a look at them to see if there is any story to tell.