I have had two LPG cars, and sadly, I won't be buying another.
To answer your question...
As I understand it, LPG is about 10% less efficient than petrol. But if it only costs half as much, then it's potentially a sensible move. But have a think about how many miles you will have to drive, before you get back that £1400 conversion cost.
Thing is, it does not always cost only half of what petrol costs, and it's now almost a lottery as to where has sensibly priced LPG.
This is not helped by the way LPG has gone up by maybe 60% in the last year. And made worse by the way that for the last few weeks, it has been impossible to buy at many many garages (and LPG garages are maybe 1 in 100 anyway - I would guess).
I love the fuel economy and I love that LPG is much less of a pollutant.
But the time (and mileage) I now spend driving round searching for a LPG garage that actually has some in stock, at a price that is still reasonable....is time I do not have to spare.
The filling of LPG is a bit startling (the first few times) as there is a small amount of LPG released when you disconnect. It's a loud HISS - which made me jump but I soon got used to it!
I have assumed you are UK based (I am in the SE UK) and what I say about availability is true for the UK. But in mainland europe, it's more like every 5th petrol station has LPG ( and a lower price than UK) so it depends where you live.
I think, these days, it is averaging out that LPG is now 2/3 the price of petrol and that ratio changes the economics of it hugely!
LPG can be £1 or even £1.10 where I live..and petrol is about £1.60 today.
It must be hard to make a decision, given the volatiity of petrol prices these last 2 weeks.
But there is something to be said for having a dual fuel car...as I could still go to work (using LPG) back in October (when there was NO petrol at many fuel stations).
The last 10 days or so, I've been driving around on shockingly expensive petrol, as I could not find LPG anywhere, at any price. But at least I can still drive to my appointments, one way or the other!
Had you considered a new (or second hand) petrol/LPG car? Rather than sinking £1400 into an existing car? Depends how valuable (financially and emotionaly) your existing car is to you...
It's not an easy decision, but to sum up, I think it comes down local availability of LPG, cost of local LPG and how many miles per year that you will do.
Good luck :-)