OK - quick update on this.
The security on our, and another, building apprehended 2 youths carrying industrial bolt croppers and a hacksaw around the perimeter fence of our building - heading in the direction of the bike sheds (as hardly any of you will know the area around DB Schenker Doncaster I'll just clarify that it's wasteland and not a shortcut to anywhere - you'd only go there if you were up to no good). They were detained, photos taken and the police called.
The local police station is 1 mile up the road. When they hadn't bothered to turn up after over an hour security had to let them go.
I went and bought my new bike last night! If the police can't be bothered to come and collect the thieves when we've caught them red handed then I really don't hold out much hope of getting my bike back. It'll obviously be too much bother for them!!
On the plus side - all the offices in our area have now created an "early warning" network. A cyclist in any office now has a list of contacts in other offices. If any of us see anybody acting suspiciously in the area then we alert the others. Granted it might not be 100% - but if it saves 1 bike theft then it's worth doing!