Hi Mark,
For spouses under the old rules (Visa application prior to 9th July 2012) Naturalisation eligibility was 3 years resident in UK. Being free from immigration time restrictions on the day of application (ILR granted ).
Spouses under the new rules (Visa application after 9th July 2012) also need to be free from immigration time restrictions on the day that an application is made.
Which means that spouses need to complete the 5 year probabtionary period and obtain ILR before an application can be made.
Hope that clarifies.