If cashing in or transferring to another occupational scheme are not options then you can either leave your pension invested in your former employer's scheme or transfer it for a . As ever there are pros and cons to both (check back through this forum for topics discussing these). Also as ever you should pay careful attention to charges (e.g. leaving your fund paid-up in your former employer's scheme could still result in a monthly policy fee being deducted through encashment of units), flexibility, choice of funds, ease of administration, implications for vesting periods (e.g transferring from one occupational scheme to another some time in the future also transfers in time served/vesting time) etc. Note that if you leave your fund in your former employer's scheme you can always transfer into another occupational scheme in years to come.
Hope this quick summary helps in some way!