I am unaware of the legalities in the UK regarding intellectual property rights, but I agree in general with GHudson on the meaning of it all.
In the USA, GHudson would be absolutely correct. If you developed something on a company computer as part of your normal job (or even as part of a special assignment), you got paid for your work already when you cashed or deposited your paycheck. If the company wants to develop it with or without you, they can - because they already own it. This happens to fall under the US Code regarding "copyrights" and "work for hire."
Here in the USA, we often have to sign a generic intellectual property rights waiver as a condition of employment, particularly if we are going to be in a creative position. Check your employment documentation (I hope you kept it!) to see if you signed something like that already.
In the UK, you would need a copyright specialist to advise you of the actual rights in question, which might differ considerably from U.S. Code on this subject. However, based on my somewhat limited understanding of USA Common Law, which is derived from British Common Law: If you sign a particular type of waiver document dealing with the intellectual property represented by the program, you will in essence waive any future claim to the program AND ITS ASSOCIATED TECHNOLOGY. Which means you would not later be able to sue them for a share of any profits they might reap from such a sale.
Further, if their corporate lawyers are worth tuppence, they will include a non-disclosure clause in that waiver agreement that will effectively bind you either for X number of years, X > 5 as a certainty, or in perpetuity. You would not be able to later re-invent this particular wheel on your own or for another company because of that agreement.
The intellectual property rights involved here are two-fold.
First, who owns the program you wrote? The company wants to own it outright so they can legally sell it without you claiming ownership (thus, the sale is called an "unencumbered" sale or a "free-and-clear" sale).
Second, who owns the design? The company wants to own the design, too, so they can prevent you from going into competition with them down the road if your relations with your company become a bit ... rancorous.