Some fairly drastic measures there Dominic!!
The weight of doubling the batteries, or carrying a leisure battery, would be horrendous.
Why not just convert to Lithium-Ion batteries? this would shed around 20kg in weight and therefore extend the range, and if you fit a removable pack similar to the set up on the new model 30, you could take the battery pack with you when you park up, and then charge it at work.
There are lots of Lithium packs available on the internet, complete with built in BMS, and supplied with charger.