I would suggest that the ECU (engine/electronic control unit) is probably the least likely thing to go wrong. Sounds like the garage are just employing the "shotgun" diagnostic system, throw something in there and see what it hits, then say it's probably that. Do they actually have any evidence to support that it's the ECU, I doubt it. ECU's are generally extremely reliable. It could of course be a bad contact in a connector, take it out, spray with cleaner, and put it back in.
Swapping coils is the cheapest simplest quasi-scientific thing to try. It may show up a problem item, it may not, but costs just a few minutes work.