It is most important that the customer keeps control of the whole process, who should tender for the work and the way its carried out.

Architects or agents often have a list of builders that could tender for the work. Do not accept all from that source, choose one or two names from your own source, if necessary contact a professional body such as, The Federation of Master Builders.(FMB) via. the internet, who will supply you with additional names to choose from. 

Make it clear that all sealed tenders should be sent direct to you, not your Architect or Agent. Once the allotted tender period has ended, open the sealed tenders in your presence and at the same time make your decision based on the information contained in each tender. 

Always use a recognised Contract form which your architect or agent can supply.

If clients follow these simple steps, it is less likely a possible 'Conflict of Interest' occuring during the tender period and a Contract Form keeps it professional.