Changing your pension fund manager on their investment returns over a short time (1-, 2- or even 6-months) does not make a lot of sense.
Also past perf. etc etc
You need to look at the management expenses and fees structures and if they are not comparable to other companies, then a change would be warranted.
As to picking a manager who will perform better than the rest over a long time-span - you may as well toss a coin. Having a range of plans with different managers is probably the best that you can hope for - as long as the management and selling fees are competitive.
BIAM have fallen from grace in know small way due to most of their key investment managers leaving some years ago. They lost a lot of years of experience when this happened and from what I hear, are trying to build up an established team again. For what its worth, Eagle Star have a good team in place and they all carry quite of lot of investment experience.