Its all set out in here - http://www.hse.gov.uk/pu...s/hsg173.htm   Monitoring Strategies for Hazardous Substances, it explains how to use the results of a preliminary survey to determine frequency of repeat monitoring and how to develope an overall strategy for monitoring

Ah sorry - I thought you were asking if there is any guidance.  When I worked for ICI we had a sliding scale of frequency based on the results.  If the result was over the WEL (only it wasn't WELs then it was TLVs - Threshold Limit Value) it meant you had to repeat immediately and then monthly for 3 months to assess if this was a true over exposure situation.   At 75% of the WEL / TLV you repeated monthly for 3 months and then reassed, at 50% of WEL / TLV you repeated 3 monthly for 12months and then reassessed. If the result was less than 50% then you repeated 6monthly and for low exposures at 10% you did it every year.  Its been 30years since I worked there so I hope my numbers are right but I do remember doing a heck of a lot of sampling!!