Hooksey,
Yeh the 40 Venom is probably too small for that combo. I use a 46 venom with bothh my 8.8 and 7.7 CReds. If underpowered sailing with the 8.8 the 46 tends to let go more than I like but it is always a sweet combo with the 7.7. I'm 91kgs and pretty heavy on my back foot so I reckon the 44 would be ideal if you are lighter or sail with less loading on fin. Also I sail in open ocean so a bigger fin does help. If sailing in flatter conditions you can go down a size as the fin stays fairly constant through the water.

PS - I actually sail the iS122 & 8.8 with a 48cm Talon in really marginal conditions and have found it to be a perfect match, but if powered up I'll change down to the Venom.

Here's a previous thread on Venoms too which has expert advice (as opposed to a weekenders)
www.seabreeze.com.au/forums/Windsurfing/Gps/C3-Venom-versus-Tectonic-Talon/

thanks guys - thats also what I thought (would anyone have one for sale?? cheap, cheap (with a thick Bali accent)??)

I can source a FO Nitro Slalom 44cm or a Meanline Slalom 44cm fin a lot cheaper than the C3s. Would anyone have experience with any of these fins and how they compare against the Venoms?

Yep as the comments above the venom 44 and 7.5 should be a sweet combo.
yer generally you can go 2cm bigger than other fins in the Venoms eg select.
After trying some talons I think they have about the same lift as the Venoms thats my personal experience anyway.
Im currenty using the Stings and are really happy with them but they are more full on.
The heavier guys will really like the Stings.
the great thing with the Venoms is the range they have. I only ever needed one fin (46) for the IS122 and 8.5 and that got me going from 10-25 knots. I still have a soft spot for the Venoms