As so often the case, not enough info. Type of engine? Mileage?

Our old Focus 1.6 Zetec had two problems causing EML illumination. The first was a failed coil pack which suddenly caused a continuous misfire and loss of power, and the second was a dirty throttle body which just needed cleaning. If it's a petrol engine I would test your HT leads for sparks first.

How could I have forgotten vacuum leak?

Probably less likely than on the ancient carburetted cars I have mostly driven, but I THINK still a possibility.

Ok so it broke down & was told the issue was the ignition coil connector needed replacing. So I sent it into the garage for them to have a look at & they said that they couldn't just replace that part as they couldn't buy it on it's own & that they needed to buy the whole thing together which will cost a bit. Any ideas as that sounded a bit off to me?

It sounds a bit off, but a bit (or a lot) off seems to be the new normal.

If the bit (Im not sure what you mean by ignition coil connector but perhaps the ignition lead. I've never had a car with "coil packs" if that's what this has) isn't available separately as an OEM/main dealer part, it may be available as an aftermarket part, or you could try sourcing it (or the whole thing, whatever that is) from a scrappy.