Thought I would just share my thoughts on this.
So I recently had my X5 converted to LPG, due to timings etc I wasn't able to change the spark plugs prior to the LPG conversion, this for a couple of reasons is something I wouldn't recommend because:-
1) The LPG will require a new remap
2) Because of below, but I knew this in advance and its what I had to do.
Over the weekend I started to put in some new Iridium plugs some my Truck can run as well as possible (so ive been told).
Looking at the engine I could tell that the right hand bank (as you look at the Engine) was going to be a pain as the LPG pipework, Vaporizer was restricting access.
Anyway on Friday night I whipped out the plugs on the left hand bank and replaced them, no problem at all and took about 45 mins.
On Saturday morning I started the right hand bank, what a nightmare, first of all I had to remove the LPG Vaporizer which was bolted down with a square security bolt, i didnt have this in my tool kit so it was a trip to Halfords and then Maplins (at least ive now got enough sockets etc for any job in the future i hope

Anyway so I got back on the job in hand, I must say it was nightmare, moving all the pipes out of the way, then fumbling around blind trying to get the furthest away spark plug out and the new one in, I could have done with a slightly smaller 3/8 extension to be fair but I worked with what I had.
Many many swear words later and stress I mangeged to get the old plug out and the new one in

The other three were a lot easier and it was almost plain sailing.
In the end it all turned out fine.
The Moral of the story is to not rush, panic or give up, just stick to the job in hand and take your time, you can do it!
If your doing a job for the first time make sure you are under no time pressure to get it finished otherwise this will add to the pressure.
So in the end my Truck runs a lot better and is giving another 3 MPG.
Never give up my fellow X5 friends!