According to the Britannic Technologies blog:
From 13 June 2014 it will be against EU law to profit from calls to post-sales lines. Premium rate, revenue-generating numbers can be used for general pre-sales lines as normal.
Where a telephone helpline is provided, the basic rate requirement means not charging more than a geographic or mobile rate. This telephone number provided should not provide the trader with a contribution to their costs.
Because of this Royal Mail have changed the telephone numbers for their enquiry helpline from 0845 740 740 to 0345 740 740§.
New products will be
1st class book of 6
1st and 2nd class books of 12
1st and 2nd Large books of 4 *
1st and 2nd x 100 business sheets *
1st Large and 2nd Large x 50 business sheets
* These will appear with 2014 year codes for the first time, producing 4 new stamps with a specific date of issue. We will produce a limited number of first day covers, so if you would like one please contact us in good time.
§ These may look quite similar, depending on the font used, so extra care will be needed when searching for the new books.
The 1st class book of 6 has been printed with M14L code and the old contact details, but we don't know of any findings yet. We don't know whether the expected Commonwealth Games mixed booklet will have new telephone numbers.
UPDATE 1 JULY:
The booklet of 6 x 1st with the old telephone number has appeared on eBay, and I have an example used from Sheffield on 20 June - any earlier dates would be useful to know! No FDCs for this one.