Ok..

Well there could be a few other things that would need to be checked - although seeing as the plugs have been changed then you can rule those out - one of things they would need to check is the ignition coil/wiring to be sure that isn't cause of the missfire - also too the condition of the fuel filter "replace anyway" and the injectors and the plugs to them - also too check that all cylinders are even in regards XX XXXXXXXXXXX as a cylinder low or lacking compression maybe due to faulty/worn valve(s) can in most cases cause missfire so a compression test would be something else to have done as well as checking the timing and the timing belts overall condition too - so the rest of the ignition system/fueling system/ would need to be checked - maybe too try one of these fuel injector cleaning additives that go straight into the tank ,you should be able to acquire these from any local motor factors like halfords for example - this should help clear out any sticking injectors etc and might help improve or rectify the fualt in particular if there is bad fuel in the tank.. "make sure too you haven't missfuelled by mistake so you'll need to double check on that"...another area to check/clean up on the throttle body and check its clear or any carbon buildup also too the idle speed control valve is clean and not sticking too / and or any intake vacuum system tubes are still in order and haven't popped off or have any splits in them/connections loose - however they maybe able to see if there are any codes logged in the ecu anyway and too if their machine has the capability then try using live data whilst the engines running and see if that can highlight anything further - so there are a few things else apart from what you have had done that can/need to be checked before you can move on to anything else deeper..