I would like to know how important your undergraduate degree is when looking for PhDs. I became interested in climate/environmental change particularly palaeoecology through my MSc but my first degree isn't very relevant. I am now doing voluntary work experience in the area I'm interested in which I hope will help. My main concern is that it has also been a while since I finished my MSc (nearly 3 yrs) so how will this affect my chances if I leave it any longer? Will it look bad that there has been such a gap? Thanks.

If your MSc is relevant then you shouldn't be too disadvantaged by your first degree. What may hinder you is the gap between your MSc and now. However you are doing the right thing about it and should try and meet and talk to as many other people working in your field as possible. You have picked a subject which is both popular and poorly funded and academics will have their pick of candidates. If they've met you, you may at least get an interview.

I've had some interviews but didn't get desired result. I think it's because due to lack of adequate experience/knowledge/average interview performance so what can I do about it? I'm doing an Open Uni course in my own time instead of their timetable as I have recently moved cities/doing interview prep. Will this matter? I am worried about the time gap as it has taken ages to get anything in this field including work experience. How much longer before this gap becomes unacceptable and how can I stand out against more recent graduates? Thanks.

If you're getting interviews, then your career history and current status with your degree probably isn't letting you down. Competition is high and if people don't like what's on your CV they won't bother interviewing you. As long as you keep working in the field then the time since you completed your MSc shouldn't be too important.

Be positive and not apologetic in interviews, if possible get feedback from the people who turned you down. Your enthusiasm and dedication to the subject should help you stand out against your younger rivals.