CPR 6.14 and deemed date of service: how do you calculate this and do both days have to be business days?

In relation to the deemed date of service of a claim form, pursuant to CPR 6.14, an opponent has argued that the reference to the 'second business day after completion of the relevant step', as set out in CPR 6.14, means that only the second of the two days must be a business day. Therefore a claim form sent by first class post on a Friday would be deemed served on the following Monday (i.e. counting one non-business day and one business day). We would be interested to hear if you have a view on this interpretation of CPR 6.14. It has always been our understanding that "deemed to be served on the second business day after completion of the relevant step" means (in the interests of certainty) two business days after the relevant step has been taken. To use the same example as above, our understanding is that a claim form sent by first class post on a Friday would be deemed served on the following Tuesday.