How Mean is my Machine?: A record of costs

I'm updating this table every time I fill up with petrol. I'm trying to do this on empty so that the (seemingly two-weekly) figures are as useful as possible. What you have below then is a record of costs as they're incurred and a moving average.
If I forget to keep the receipt, I'll estimate and inevitably there is some rounding.

Date	Description	Amount	Mileage
16 Aug	Deposit	£100.00	65,039
17 Aug	HPI check	£6.99
19 Aug	Insurance (12 months)	£191.98
20 Aug	Purchase	£600.00
20 Aug	Petrol (filled it up!)	£34.17
20 Aug	Tax disc (6 months)*	£68.75
6 Sept	Resident’s parking (12 months)	£30.00	65,196
14 Sept	Refund!	-£700.00
17 Sept	Purchase of ‘new’ car	£995.00		Start again!
17 Sept	Credit card charge (grrr)	£20.00
17 Sept	*But price includes 6 months tax (hooray!) so refund due...
17 Sept	Refund on insurance (hooray!)	-£25.12
	So fixed annual costs = £789¹ or			£15.17pw
17 Sept	Petrol (filled it up...)	£50.07	68,567	Approx 43L
	382 miles in two weeks =	*£40pw*²	or	*21p/mile*
	*615 km*			*13p/km*
	Fuel consumption 615km/43L = 14km/L OR 382m/9.46gal = 40.48mpg
1 Oct	Petrol – another fill-up	£50.00	68,949	43.14L
	*370 miles in 14 days/2 weeks =*	*£40.19pw*²	or	*22p/mile*
	*595 km*			*14p/km*
	Fuel consumption 595km/43.14L = 13.8km/L OR 370m/9.49gal = 38.99mpg
15 Oct	Petrol – yet more	£49.50	69,310	42.71L
	*346 miles in 20 days/3ish weeks =*	*£31.67pw*²	or	*27p/mile*
	*557 km*			*17p/km*
	Fuel consumption 557km/42.71L = 13.0km/L OR 346m/9.40gal = 36.82mpg
3 Nov	Petrol again!	£48.20	69,655	41.59L

1: I’ve taken half of the purchase cost because I don’t
think there will be much residual value at the end of two years. For those of a
non-accounting disposition, this is ‘straight line’ depreciation at 50% per
year so at the end of two years the car will have no capital value. If there is
this might contribute to maintenance costs. All other costs are annual.

2: And that’s a scary figure when you consider it’s out of
taxed income – so you’d have to earn about £50 to pay for this! Oh and it
doesn’t include any maintenance, tyres etc though I’ve tried to account for
depreciation.

1 litre = 0.22 gallons. 1 miles = 1.609 km