What you should also consider is that abroad, LPG is much more difficult to obtain compared to petrol. Also parking garages more and more (at least over here) LPG powered cars. Next driving LPG means standing at the petrolpump around every 200 - 300 miles, where 200 is more realistic. (LPG tanks contain only 80% of their max contents, a 13 gallon tank fills up to only 10 gallons, which in your case would be 250 miles.
As dutch resident I have an Octavia II, 1.6 on LPG. my tank is 13 gallons, but around 225 miles(around 22.5 mpg), my tank is bordering on empty, and I need to switch to petrol. On petrol BTW, i use 33.6 mpg.
So for comparison i deem it better if you would use real numbers, instead of the estimates from the guy trying to sell you the installation.
My experience is also that the more power is asked from the motor (mountains, trailer) LPG use will rise more than petrol use. LPG burns hotter and will damage your engine more than petrol, however if you decide to sell it again at 75000 miles, you will not notice the difference on that terrain to much
Here the taxes for LPG are bigger, putting the switchpoint typically around 7500 miles per year. If you drive less LPG is more expensive over here.
I do not know about taxes in the UK, but if it makes a difference, you should also take it into account.
Just for information: Here in the Netherlands LPG can almost only be obtained outside the city. You should also take into account the extra mileage you will need for filling up your LPG installation if you cannot fill it up at your usual petrol filling station(s).