Hello Colin.
NGK are a good make of spark plug. I cannot say they are the best for your vehicle, but they should not fail as drastically as described. LPG should not burn hotter than petrol if correctly mapped. It has a higher calorific value and RON which makes it burn longer.... The temperatures that Petrol and LPG create whilst being burned within the cylinder head could not melt a spark plug. Something more drastic has happened in my opinion, such as the cylinder was running lean or rich for too long. There is also the possibility of the plug not being fitted properly in the first place. If the plug was able to draw in air, you will have a rich mixture burning at all times.
Once again, I wish you the best of luck.
Kind regards
Leon
Profess LPG LTD.
EDIT 1: 09:00 21/06/13
Take a look at this:http://www.lpgforum.co.uk/viewtopic.php?f=28&t=12342
Is this anything like your situation Colin?.
Does it look like your plug?
These images are not owned by me and I take no credit for posting them here.
Credit goes to the owner of these images 00lewisd From http://www.lpgforum.co.uk/viewtopic.php?f=28&t=12342.
Hope this helps you determine the reasons as to why your sparks went the same way as this 1 did.
Edit 2. 08:00 22/06/13
It is also advised to look here for support from NGK.
http://www.ngksparkplugs.com/tech_support/spark_plugs/p5.asp?mode=nml