Here's some information I found on this subject.
Note: On 7th February 2005, the British Home Office announced important changes to this working holiday visa.
The old system: under the old working holiday scheme, visa holders could work in the UK for the entire 2 years of their stay.
What's new: working holiday makers can only be employed in the UK for a total of 12 of the 24 months duration of their visa. You can work for twelve months straight and travel for 12 months, or you can work for several different periods of time in the UK as long as the total does not exceed 12 months.
Note on Type of Work: it does not appear that the changes have restricted the type of work that a working holiday visa holder can undertake. While you must take work that is only ‘incidental to your holiday’ there is nothing in the changes to say that this work cannot be in your profession (as per the pre-June 2003 rules stated).
These changes came into effect on 8th February 2005. If you held an approved working holiday visa before 8th February 2005 but had not entered the UK yet, you will enter the UK under the old rules (i.e. you can work for the entire duration of your visa, there is no restriction in that regard). If you have submitted an application but have not yet received approval, you will be subject to the new restrictions and be able to work only for one year in total.
http://www.workgateways.com/working-uk-visas.html
And some additional information:
Extension of the upper age limit on the Working Holiday Visa from 27 years inclusive to 30 years inclusive.
Removal of restrictions on type and duration of work undertaken in the UK.
Holders of a UK Working Holiday Visa now have an option of switching to work permit employment after 12 months, upon meeting certain criteria.
http://www.workgateways.com/working-visa-faqs.html
Although, unhelpfully "certain criteria" is not defined.