No guidance has been issued since January 2008. The width and length of parking bay markings is prescribed in regulations and detailed guidance on their use is contained in the Traffic Signs Manual, Chapter 5 - Road Markings (2003). This document is on the Department for Transport's website at the following address:
www.dft.gov.uk/pgr/roads/tss/tsmanual/trafficsignsmanual chapter5.pdf
We have received no formal proposals for changes to regulations or guidance. However, from time to time we do receive applications from local highways authorities for authorisation of narrow and/or shorter bays where exceptional site conditions prevent the use of prescribed markings.