I've soaked in a lot of information today and feel like I've found a few keys and unlocked a few doors!
When I went to photograph cars a couple of months back at Donington I didn't have my Camera set to Al Servo mode, which probably explains why it was losing focus on the cars when I was tracking them! At the time and until today I kept thinking that it was because my Camera isn't good enough and the 55-250 wasn't a good enough lens.
I guess that would go some way to explaining why i was struggling to keep the focus locked on the cars then! Also only found out about the centre point red dot focus yesterday and now thanks to your post and other reading on the Internet across various forums have discovered how AV and TV modes work.
99 percent of my photography so far has been landscape in manual mode which I think I have a decent understanding of, but action and sports photography is something which I have only just started sinking my teeth into.
I quite like the idea of keeping it in AV mode for the fast jets, at F8 or so (seems to be a lot of people's idea of a good sweet spot and I've noticed decent DOF and sharpness, hence sweet spot at this aperture in my landscape pictures as well)... then most likely switch to TV mode for props at about 1/320 (until I improve and have scope to go lower).
Hope I make sense!