Just to provide an update on this.
I finally got around to unlocking my Japanese iPhone using a provider on
eBay which was quite trouble free apart from having to wait a few weeks for the relevant unlocking code to appear (which is advised at point of sale).
It works perfectly and even with the cost, is worth it for us as it means my mother gets a 'new' barely used iPhone - for much cheaper than buying a new one - and a big upgrade on her old one which was on its last legs
I would say, that I am sure these firms have a healthy mark-up but there are times such as in my case that you can't get a network to unlock a phone and/or its impractical to do so.
Certainly if it was a UK phone, I wouldn't have gone down this route but would have been able to go direct to the carrier but mine was locked to a Japanese carrier and the rules re: unlocking were different there..