Possibly a better way of looking at this is to take mileage into consideration
Gas - 200 miles for £20 ie £0.1/mile plus conversion cost of £1200
Petrol - 143 miles for £20 - say 35mpg best average from my 16v ie £0.14/mile
Break even point is where these two meet on a straight line graph of cost against mileage for fuel consumption ie where the cost will be the same for both
Cost to run gas = £0.1/mile x mileage plus £1200
Cost to run petrol = £0.14/mile x mileage
Where both costs are same
0.1 x mileage +1200 = 0.14 x mileage
Mileage = 30 000
In other words - to ensure the conversion is financially worthwhile, you'll have to run the car on gas alone for a further 30 000 miles. After that, you are quits in, get rid of the car before then and you lose out.