Thanks guys
The tailbag was from Hein Gerricke and was £10 reduced from £29.99. However, it wasn't big enough for a helmet to fit in. I took it back this afternoon and once I'm happy with fitting a tailbag to the bike I will buy a bigger bag.
I was amused today though. I went to a bike dealer and asked about fitting a tailbag to a SV. The guy was good enough to show me how he'd fit one to an SV1000 S (no other SV650 in stock) and the thing was all over the place. The first time wind hit it, it would be off - probably into the wheel!!!