Hi Tim
It can be very difficult for smaller sole traders or partnerships to win some of the public sector tenders eg schools, council buildings, etc. Anything over a certain value, has to be openly advertised in the Official Journal of the European Journal (OJEU) which many companies keep track on through the websites already mentioned. This means you can often get in excess of 100 contractors applying for it and is generally difficult for smaller contractors to compete on.
Again, talking generally, bigger clients offering bigger contracts do prefer dealing with Limited companies because it is easier to check their accounts to make sure they have the resources to deliver the contract. To get into some of the public sectors contracts it's often easier to try and do one off bits of work for them first and build your contacts like that.
Regards
Mike