There is information in the HSE guidance, HSG 173, "Monitoring strategies for toxic substances"

It is addressed primarily to employers. First published as EH42 but revised to increase accessibility to non-specialists and so enable wider use of it as an introduction to the design of inhalation exposure monitoring strategies. Accordingly, the advisory procedures offered here are designed to facilitate investigations into the nature, extent and control of hazardous exposure.


http://www.hse.gov.uk/pubns/priced/hsg173.pdf

There is also "Controlling airborne contaminants at work, A guide to local exhaust ventilation (LEV)"
It icludes the principles and good practice of deciding on, designing, commissioning and testing cost-effective LEV.The guidance is written for suppliers of LEV goods and services, but will also be helpful for employers and managers in medium-sized businesses, and trade union and employee safety representatives. All of these groups need to work together to provide, maintain and use effective LEV and to reduce exposure from inhalation of hazardous substances.

http://www.hse.gov.uk/pubns/priced/hsg258.pdf

Soldering: Hand-held with lead-based, rosin-cored solders
http://www.hse.gov.uk/pubns/guidance/wl17.pdf

I very much doubt that you will be able to undertake the air monitoring yourself unless you go for additional training and gain the competency--then also you will have to invest in the sampler pumps , tubes etc and send the tubes for analysis and then calculate the results--It is likely to be more cost effective to get the services of an Occupational Hygienist. The BOHS website has details and they are likely to be on the OSHCR

http://www.bohs.org/OHServices-directory/


http://www.oshcr.org/