Thanks for pointing me in the right direction dimsav.
This is from rule 287:
(ii) the applicant is still the spouse or civil partner of the person he or she was admitted or granted an extension of stay to join and the marriage or civil partnership is subsisting; and
(iii) each of the parties intends to live permanently with the other as his or her spouse or civil partner; and
(iv) there will be adequate accommodation for the parties and any dependants without recourse to public funds in accommodation which they own or occupy exclusively; and
(v) the parties will be able to maintain themselves and any dependants adequately without recourse to public funds; and
(vi) the applicant has sufficient knowledge of the English language and sufficient knowledge about life in the United Kingdom, unless he is under the age of 18 or aged 65 or over at the time he makes his application.
Based on this, I reckon we will be fine to leave the UK for 3 months per year, providing we demonstrate that our home is in the UK. We will show that we have our own house in the UK, where only we live, with our name on the bills, for the full 2 years. We will also show other proof of permanent settlement - jobs, bank statements, childrens schooling, photos, that sort of stuff.
I had been concerned there may be a draconian rule on maximum number of days spent overseas, so I am relieved that is not the case.
My overall experience of the visa process is that it is demanding and time consuming but providing you take the time to gather all the evidence to prove that it is a truthful application, the immigration officer will see that the application is genuine and appropriate and will approve it.