The standard position is -
If a "final salary" pension is left in a previous employer's scheme it is called a deferred pension (often wrongly referred to as "frozen"). Some or all of it (I'm a bit out of touch - it's probably only the part called the GMP element) receives annual increases in the years before retirement.
If the "normal retirement age" of that scheme is 65, then no pension will be paid from it until that age (i.e. you can't claim an early pension from it).
At any stage you can move the transfer value into another approved scheme - a bank or pension company or to a new employer's scheme. At each stage (transfer from, and transfer to) each organisation is likely to skim an administration charge from the transfer sum. One advantage which may exist with a transfer to a pension company is that you may be able to get an earlier pension than age 65 (obviously your other finances may not allow this). If you transfer it to a new employer and then leave that company you have the same dilemma again (and more charges).
One important factor is your age. If you are near to retirement, it may be best to leave it where it is now (avoid paying the administration charges). If you are young, it may be best to transfer it to a pension company and let interest boost it over the years.
If you decide to transfer it, act soon after the decision. If you delay for, say, a year it will cost you dearly later on. (£10,000 transferred to a pension company and left for 30 years may grow to £100,000. If you delay and it is there for only 29 years - at the same rate of interest - it would only grow to £93,000.)
I'll make no comments regarding your old scheme if it is a "cash accumulation" type instead of a "final salary" type - I just don't know much about those.
Try to find independent advice - and be careful about trusting anyone who has a policy to sell.
Twiglett - I'd appreciate your thoughts on these comments.