Does it happen all the time or just when it's cold, does the eml come on at all. As mentioned leads and the coil pack could be playing up but that usually brings up the eml on your dash.
Yeah, like most lupo conversioins, you have to lift the loom/drop the new in. If you have it and you were willing to give the engine a go yourself it really isn't much work and as long as you're not an absolute retard it isn't hard. Keep your box though.