In a worst case scenario, should the pension scheme go bust – the government will pick up the tab, but only paying you 90% of what you would have otherwise received, and this amount would not be indexed linked, once you retire. That is, whatever your fist monthly pension payment is – this will be the same for the remainder of your life.
Your employer should be taking steps to reduce this pension deficit. This will normally involve them paying extra into the fund – most companies in such a position will also reduce other benefits that were offered with the pension. Such as paying out much less to those opting to retire early, and limiting the inflation proofing to a lower maximum amount (say 2%, when inflation is currently running at over 5%).
Remember that once you reach retirement age – you can take up to 25% of the notional pension fund required to pay-out your pension (tax-free). The required fund can be 20x your annual pension – so if you had a £10k pension, your fund could be worth £200k, 25% of which would be £50k. Of course, in the above example having taken a £50k tax-free lump sum – your annual pension would be reduced by 25% to £7.5k.