I got around to removing the plugs from my bike and found that they are NGK D8EA, with a gap of around 1.2mm! Time for new ones. The BMW literature and the Clymer say that the correct plug is a D7EA or DR7EA (only difference is the resistor), but it seems that many are using the 8 heat range. What I would love to know is why, is there a reason, perhaps something written somewhere, or are they just easier to get hold of.

Resistor plugs should be used on 4V models, due to different coils than the 2V's.
Can't tell about the heat grade, I've always used D(R)7EA's.

On my 8V K100RS I've been using NGK DP8EA sparkies. One reason is that the 'cooler' heat range of 8 as compared to 7 seems to work in the hot Australian climate, and the P in the part number means it's a projected tip, which helps in putting the flame further into the combustion chamber, but the slight additional length on the ground electrode doesn't interfere with the pistons. I've been doing this for three years now. I agree that you would not want to use a resistor plug in the 8V motors. I did once and was talking to Charlie99, he warned me about it, as the bike seemed mildly slower to rev and didn't return decent fuel mileage. Swapping them out for non-resistors instantly improved the rideability.