Elizabeth, the main thing is that your pension will not disappear like those unfortunates, including my uncle, who saw their pension completely disappear when Robert Maxwell died.
There is a Pension Regulator who can require the company behind the pension scheme to pay money into the scheme to rectify the deficit. If the company does not have the money one of two things are likely to happen.
1) the company goes bankrupt and your pension is vested in the Pension Protection fund. As you are already drawing a pension that will remain unchanged but the annual increases you receive may not be as generous.
2) If the company is a good company and profitable, it may be sold, but the new owner will refuse to take on the pension scheme. The sale price will be paid into the pension scheme and it will be taken over by the Payment Protection Fund as before.
Here is the web site for the PPF. www.pensionprotectionfund.org.uk/Pages/homepage.aspx.
The main thing is that your pension will not disappear. There is a safety net.