I have just laid a whole run of pipes floor boards on top and just read I should be using 40mm for waste from the shower - I have not laid the correct size and have only used 30 mm will this be a problem ? (please tell me it is) if not I will change it- but it will be more work and the wife will have been right all along (they always are anyway )!!

Yes, a problem, what with showers and tubs clogging so easily with hair, and it's probably not code in your country anyway.

How long is the drain line? 30mm is really small for a drain line and 40(1.5+) is roughly what is used in US and CAN. At this point, I'd probably let it go as long as you can. I'd make sure to keep water flowing to push stuff through and hopefully you put in sufficient slope.

In my area, Code mandates that we use 2" pipe ( 50mm) pipe including trap.