I would think very hard before retraining in employment law in a large firm.
Firstly, as others have said, trainees are very much cannon fodder in the larger firms. You will also get to spend a maxium of six months in employment out of the two years and few firms will guarantee you a seat, as it's very popular. You could end up doing the whole two years in other areas.
Even if you get a seat in employment, there is normally stiff competition for the NQ positions (if there are any) and external vacancies for NQ positions are rarer than hen's teeth.
Once you're qualifed, bear in mind that you're dealing with people at a moment of crisis a lot of the time. That means that their "this is a disaster, must stay late all week and sort it" is your everyday life. You just jump from person to person's crisis! There is massive pressure on hours in a grinding day-in day-out kind of way (as opposed to coroprate, say, where you might work til 4am a few nights, but then have a lull before the next deal).
If you're really set on this area, I'd suggest going for training contracts at niche firms. They may or may not pay PgDL and LPC fees. Also, think about whether you want to work for individuals or companies, as niche firms tend to slant one way or another.
To be honest,I'd probably think more broadly and perhaps try to become more specialised/senior in HR itself. As an employment lawyer, we are under massive pressure from in-house teams and HR consultants, who can do work at much lower fees. I think that you could do similarly interesting work (unless transactions/deals are your thing) at a large PLC in HR. There is a great deal of overlap and good HR directors can handle many of the same pieces of work that I can. To be brutally honest, I toy with making the leap the other way.
Sorry. Don't mean to sound negative or bitter. Having a bad week!