I know nothing about Raleigh-brand cyclo-computers - mine is a VDO unit with wireless speed and cadence.
The obvious thing to check if you're not getting a speed reading is that the coupling to the wheel magnet is good, based on the alignment between the magnet and the pick-up (probably mounted on the fork, I would assume). It may be easier to check this "off the bike", by just passing the magnet across the face of the transmitter while holding it in your hand. This also lets you do this at varying distances from the main computer, to rule out weak transmission between the pick-up/transmitter and the main computer.
Some wireless units use coded digital transmission techniques between the pick-up/transmitter, and the main computer. If this is the case, there may be a "synch" or reset process that you need to go through, to train the computer to respond to the transmitter (basically ensures that the hand-shaking between the two is correct). (The coding scheme is used to prevent the computer from listening to someone else's transmitter, instead of yours. But if the two aren't synched properly, your computer may not be listening to any transmitter.) (Units with analog transmission don't have this problem, but may be more subject to interference from other bikes, etc.)
Finally, it doesn't hurt to remove the batteries from both pieces, and start them both up from scratch.
