I'm Sorry You've Lost Me, Like I've Said I'm REALLY New At All of This....

You simply replace the low pressure sodium ballast with a florescent ballast, basically a low pressure sodium tube is the same as florescent tube electrically, but instead of using Mercury to generate light they use Sodium, so they will run on the same gear and you can use cheap ballasts like Fulham workhorse ballast, the WH-3 will power an 18 watt SOX with no major issues, infact they are more efficent then a normal LPS ballast see 589's tests of it HERE and see it in operation HERE and if you want to run higher wattage there is larger florescent ballasts easily available today.

Also if you want to know how to hook it up or what is the best ballast to use with a LPS tube just ask on the forum, several people will chime in point you in the right direction.. 

As a result, we've spent more money than we ought to, we've bought loads of lamps and NO Christmas presents for anybody (well not many..!) 

Same here! My Visa is looking rather loaded...now to pay it off, but that will only happen if I stay off of eBay.

I Can See This Getting To Be a Problem, But Only a Light One......
Have Already Bought Another Bulb But It Probably Will Need it's Own Thread...

Especially after you post a picture. (I've lately obtained quite a few NOS Westinghouse lamps, and will soon post pictures of them...some are quite rare and most I obtained for free ...can't beat that.)