Next week (12th) Lidl doing a heart rate monitor for £15. I can't comment on the quality but I expect others have tried these in the past and can relate their experiences.
http://www.lidl.co.uk/cps/rde/xchg/lidl_uk/hs.xsl/index_29368.htm
http://www.lidl.co.uk/cps/rde/xchg/lidl_uk/hs.xsl/index_29368.htm