Genghis, I think you may have the answer there. However, if you have over 1,000 slides to do, then you might find that soimething like the Nikon Coolscan or another dedicated slide scanner might be economic.
The reason that I dodged your question earlier, is that of many threads that I have read on the aforementioned photo forum. many users have stated major frustrations with quality and colour management. Even though I do a fair bit of scanning, I will make my excuses when faced with slides.
A plus with a bureau service, is that they will be responsible for providing a level of quality that is quite good (one hopes) but if they ain't - you can go back to them and complain.
£30/100 is cheap. Think I know which I would do.
Conan