This is from a page regarding rules in Ireland,not sure if any different to here.
//Sunday work and jobseeker's payments
Since 21 February 2013 Sunday working is taken into account for Jobseeker's Benefit. You must be unemployed for 4 out of 7 consecutive days (including Sunday). If you work on Sunday you are considered to be employed for 1 day and 1/5th of your normal rate of Jobseeker's Benefit will be deducted from your weekly payment.
Even though the social welfare employment week now takes Sunday into account you are not considered unemployed for 7 days. You are considered to be unemployed for 6 days out of the 7-day social welfare employment week. This means that for each week that you are fully unemployed 6 days are deducted from the days remaining on your Jobseeker's Benefit claim.
In the same way a person who works 2 days a week would have 4 days deducted from the days remaining on their JB claim.//
http://www.citizensinformation.ie/en/social_welfare/social_welfare_payments/social_welfare_payments_and_work/jobseekers_benefit_and_work.html