So after my journey to and from the factory the figures are in:
Travelling to the factory
Distance: 236 Miles
Fuel Used: 26.5 Litres
Brand: Shell V Power (99 Octane Rating) @ £1.45 per litre
MPG: 40.5 MPG
Travelling from the factory
Distance: 208 Miles
Fuel Used: 20.6 Litres
Brand: Esso Supreme+ Unleaded (97 Octane Rating) @ £1.40 per litre
MPG: 46.1 MPG
The journey time was longer coming from the factory due to an accident on the M25, but traffic flow was much faster on the M40 coming home.
Looks like I may be leaving vpower!