If you combine them, the old scheme becomes part of the current scheme and is subject to its rules. You will not have the option of maturing it early or at a different time. Also, if you died pre-retirement, the benefits under old pension schemes are paid out as a lump sum. For current schemes, the lump sum is limited to 4 times salary and the remainder must be used to purchase an annuity (no idea why this rule is in place, it only applies to active company paid pensions and none others).
Steven
http://www.bluewaterfp.ie (www.bluewaterfp.ie)