Anyway, with many thanks for complete help to Fluffcat1(there's no way I could have done it without), I disassembled the phone and cleaned the keypad contacts and it works again.
I followed Richardís already-posted instructions on how to take a C600 apart. The method for the SP5 differs slightly and he suggested I post instructionsÖ.so here goes. Iíve copied most of Richardís instructions but changed the bits that are different between the C600 and the SP5.
As guru Richard points out in his post: DOING THIS WILL INVALIDATE YOUR WARRENTY. ITíS ONLY A GUIDE AND SHOULD ONLY BE DONE IF YOU REALLY HAVE TO AND AT YOUR OWN RISK. But it could be useful if you need to replace parts, including damaged facials.
You need a Torx T5 screwdriver, a small flat blade screwdriver and a Philips 0x50 screwdriver (you can get a set with all these in from Maplins for about £10). Also a plastic tool to prise open the covers (I used an old credit card) and some Isopropyl Alcohol (also from Maplins) to clean anything inside you want to.
1. Remove back cover, sim, storage card and battery.
2. Remove the plastic aerial cover (the top bit of the back of the phone) To do this, remove the rubber external aerial connector bung. Put the phone screen -down on a surface. Youíll see a hole in the top right-hand side of the battery compartment. Put your flat blade screwdriver in the hole to release the clip holding the aerial cover. Then use your plastic prising tool to prise the cover outwards and upwards. Eventually youíll find it will slip off upwards.
3. Then remove four chassis screws, one in each corner of the back of the chassis, and the two mainboard screws in the battery compartment. Youíll need the T5 screwdriver to remove all these.
4. Then use the plastic prising tool to prise apart the chassis case from the front facia. It takes a bit of wiggling. I found it best to start at the top of the case, but eventually youíll release the clips holding the chassis case to the facia. TAKE CAREÖTHERE IS SHORT RIBBON CABLE CONNECTING THE MAINBOARD TO THE CHASSIS CASE. When youíve prised the two apart just enough, put the opening tool in and gently lift the ribbon cable connector from the mainboard.
5. Then remove the mainboard. First, using your plastic tool, gently lift off the ribbon cable connector connecting the mainboard to the keypad. The mainboard is held onto the front facia by two plastic clips, one on either side.
6. Once the mainboard, with LCD attached, has been removed, you can clean dust etc.
7. To remove the keypad for cleaning etc, put the front facia face down. Youíll see four screws at the back of the keypad near the edges. Remove these with your Phillips screwdriver. There are four more screws on the plate at the back of the joystick. Remove these with the Phillips screwdriver. Youíll now be able to prise the keypad from the front of the chassis, but be careful not to tear the ribbon cable. You can now gently peel back the white plastic keypad backing and clean the connectors.
8. To put it all back together again, reverse the procedure.
Iíll try to add some pics to this when I get a chance. Hope it's of use...and again many thanks to Richard (Fluffcat1)