Help please with ignition coils

[MistralS60]

"Ignition coil/s failing (highly unlikely and extremely rare on P2 cars with OE coils) but when a coil fails, it always does so under load or when hot!" (LeeT5's post elsewhere.)

Please may my much-loved 2004 S60 2.0T auto ask how coils fail ? Do they stop working completely or just reduce the HT voltage ?

Said S60 has just passed 118000 miles and runs perfectly at all times to my old but experienced ear, but the MPG around the hilly, bendy local area has dropped from average 24 to 22.

I know the weather's cold but the engine always gets fully hot and plenty of load uphill . I don't know if they're OE coils. The car has been serviced regularly by an expert friend who has never billed me for coils.

IIRC the single coils of the "good old days" used to age and need replacement, and I wondered if this applies here.

If so, can anyone please advise the best bang for my bucks ?

Cheers
keith