Nick
if you register the Seneca Five twin under 2000kg ie 1999kg then no navigation charges and often cheaper landing fees.
On the Seneca count on 26 USG per hour fuel.
With any aircraft dont fool yourself on the costs look at the direct and fixed costs and make sure you are using the hours to make the whole thing viable.
Unless you are churning out the hours ie 200 to 300 per annum you may find it cheaper sharing or leasing.
On something like a Seneca five AND BEING REALISTIC not head in the sand £400-450 per hour all in on 200 hrs per year.
renting or head in the sand £300-350.
Pace