How your pension is protected depends on your pension type.
If you have a money purchase/personal pension then there is a risk you could lose money if investment performance is poor. This isn't covered by a compensation scheme unless it results from bad/dishonest financial advice, in which case you might have a claim against the adviser.
If you lose money due to a pension provider going bust (unlikely, and even if they do your pension should be ring-fenced) then 90% of any losses would normally be covered under the Financial Services Compensation Scheme (FSCS) if you have an insurance style pension such as stakeholder or personal. SIPPs are normally categorised as investment pensions which means FSCS cover of up to £50,000 per investment firm within the SIPP.
If you have a final salary occupational pension and your employer goes bust without enough money in the scheme to pay your pension in full, you should be covered by the Pension Protection Fund (PPF). If you've already reached the scheme retirement age your pension is fully covered, otherwise protection is 90% of your pension subject to annual cap of around £30,000.
Click here for more information.