Vehicle Data Sticker(1) and the Vehicle Registration Certificate(2) and the HPI report itself (3) are your best bets I would say...
1) The Vehicle Data Sticker is usually affixed to the inside front-cover of the service record, and another copy inside the spare-wheel well. This lists all softs of information about the vehicle - there'll be a "Sales Type / Model" field somewhere toward the top of it - it's normally six characters, and will state exactly what model, configuration and trim level the car is. Additionally (and probably more simply) on this sticker, you'll have the engine and gearbox codes, if it's a C5 RS6 (the first one), it'll have BCY and an RS6+ BRV. I couldn't tell you the gearbox codes off the top of my head. It'll also show the full vehicle identification number...
2) ...Which should marry to that on the V5, which should also indicate that the model is RS6/RS6+/plus.
3) ...Which should marry to the HPI report confirming that the other paperwork you've seen is genuine and accurate.
It's fairly common for insurance 'poke your reg in here' boxes to be somewhat misinformed, particularly with models that never sold in massive numbers; very often they just get lumped into the next nearest thing which does have a category set-up. Hopefully this is just another case, but using the 3 bits of paper above you can get things a little clearer...
All the best,
Rob.