Hi Pinklips - I have been in this scenario myself. I am a nurse and my Trust offered to Second me for a year to get my Masters in a further qualification - it would only have been a year at Uni though so much less time than you are having to consider. It was really bad timing in my eyes as it meant starting the qualification a few months after me and DH got married and we had always planned to TTC straight away.
I was 30 and thought, "Do I want to delay TTC by a whole year?" - I also knew that after getting the qualification it wouldn't make sense to me to then TTC and leave my new profession straight away after working so hard for it. Ideally I would have liked to have worked in it for a few years before then making any life choices that would take me out of it. But that would mean delaying TTC for 2-3 years and I knew that could lead to problems such as it being harder to get pregnant the older I got etc.
I was really torn but in the end I turned down the Secondment. Me and DH got married, we started TTC 3 months later and I'm now 6 weeks away from giving birth. I know the offer of Secondment may arise again in the future or it may not, but I know I made the right decision for me. I've got 25 years ahead of me yet in my career and I certainly don't feel I made a mistake in making the choice I did.
However, your situation is very different to mine as your Secondment length is much longer and you have already been trying to conceive for quite some time. You don't say how old you are so I don't know if age has to factor into your decision but definitely was for me.
If I were you, I would do the Secondment. I think you should do research into what would happen if you did become pregnant during your training as it could potentially get very complicated and you need to know it is a situation you can deal with should you get pregnant before committing yourself. Would you aim to return to your studies ASAP after the birth or would you want a few years out?
In a perfect world the best choice would be to delay TTC (if that was an option for you) get your qualification and then start TTC. But life just isn't that simple is it?? 
what does your DH/DP think??