Ok, trying to chips a diesel we all know in many cases there are fairly large gains to be had. Having a look around the internet it's really hard to know who to trust.

There is a lot of this kind of thing:

http://www.chipexpress.com/diesel-tu...i-kit-451.html

Now just looking at the plugs Im inclined to say it goes inline with one of the sensors rather than being a piggy back ecu or anything worthwhile?

This sounds more like a piggyback ecu one might have seen on petrol engines not so long ago..

http://www.torqtec.com/?ma=operation

And finally Stu et al (and no one else Ive been able to find) offer a chip fitting service akin to the chip tuning I was aware of from petrols years ago:

http://www.evolutionchips.co.uk/chipsearch.asp

My question is has anyone had any experience with any of the above, are the external boxes just like the ebay resistor mods or are they more like a proper piggyback ecu? Which brands do people recommend?

Cheers