I've found the part where this is mentioned!! Family Policy (Version 14).
"If the applicant has already completed 60 or 120 months in the UK with limited leave as a partner, they should be informed that, should the reason they do not meet the requirements for indefinite leave to remain be overcome, they will be eligible to make a further charged application for indefinite leave to remain at any time within the 30 month period of leave granted under paragraph D-LTRP.1.2. or D-ILRP.1.2. They do not need to wait until their leave expires if they become able to meet all the requirements. If not, they should make their next application no more than 28 days before their leave is due to expire, or no more than 28 days before completing the period of leave in the UK required for them to be eligible to apply for indefinite leave to remain."
Looks like that answers it - you can apply for ILR any time once you have been in the UK at least 5 years and can meet the eligibility requirements.