If served by royal mail with signed-on-delivery and signed at your current address, it is considered served, even if it wasn't you that signed for it.

Allegedly ( ), the way to avoid the fine is to wait until the last possible moment, then write to them to appologise for not paying the fine sooner, but you have been out of the country. Keep working along similar lines avoiding a summons but also avoiding paying the fine until 6 months has passed since the original offence. Once six months has passed, they can't prosecute.

Other ways to avoid apparently include insisting on seeing the evidence including both the photos with actual timimgs printed (and check that the timings are to the required accuracy) and requesting a calibration certificate with a date within 6 months or 1 year (can't recall which).

If in a 30mph speed limit, check the spacing between lamp posts is correct (quite often they get installed to the wrong spacing) and in all cases, check that the speed limit signs and repeaters are in place, made of the appropriate material and not defaced.
Also, check that the speed limit * 10% + 2mph is less than the speed you were clocked at and demand proof that the camera was correctly calibrated at the time if not.

Finally check that the camera itself is normally clearly visible and has the yellow high-vis panels stuck on it and that their are speed camera warning signs within 2 miles of the camera along the route you took. These last few won't get the charge dropped on their own, but can help your case.