Anyone know if the timing could be adjusted back easily enough and what a garage would normally charge in hours- ie 2 or 3 hours?

Bit more info: car was running rich and seemed to drive fine. Idle was a bit poor but I had it down to iscv or such. Would always splutter first thing on a morning and generally die within 30sec of starting.
Drove into multistorey car park at metrocentre fine but when we came back it wouldn't start. Changed coil- no difference and checked plugs which were black and sooty. Wiped down with clean polishing towel and replaced. No change. Rang AA and guy came out and undid boost pipe at the throttle body, held the butterfly open a touch and asked me to turn it over. Had sparks out throttle body which he said is backfiring and most likely to be timing out. Should I just try changing plugs and see if it runs for now- cheapest first. I only changed plugs and leads in November or there abouts though....

Any other suggestions or anyone agree?