What should I take into consideration when deciding whether to transfer a pension fund?

It's complicated, and if the sums involved are significant you should seek (and pay for) professional financial advice. I am not a financial adviser, and therefore this is not financial advice.

If the previous pension is a defined-benefit (final salary) pension then you're very unlikely to be able to transfer it into a new scheme and get the same benefits -- you'll only get a lump-sum transfer that can be used for a defined-contribution scheme, and the lump sum is unlikely to be large enough to get the same benefits. So leave it where it is.

If it's a defined-contribution (money purchase) scheme, then it really depends on how much they'll charge you for the transfer, and how much the old and new pension providers charge in management fees. Read the small print for both schemes. It's also possible that the two schemes don't offer the same range of funds to invest in, and you should make sure that the new scheme offers a suitable fund for you.