Dave, it depends on whether you hold a Part-FCL licence or a national licence issued under the ANO. At the time of writing, any instruction by Part-FCL instructor certificate holders requires at least a Part-MED Class 2 certificate.
If you need to revalidate a currently valid CRI certificate included in a Part-FCL licence, you can do so without holding any medical - but you can't exercise the privileges unless you hold a valid Class 2 medical.
AOPA has recommended to the CAA that a CRI should be permitted to exercise his/her privileges with a PMD. Hopefully we'll know more in the New Year!