Oh what a Beauty. Please take care of the gem or hand it over to me.
What do you need to do except of cleaning the pots and changing battery.
20£.... You won't get one for 200 today.
The bass is from 1980 and for sure made in Japan.
Please be careful . Do not let anybody rip you off by telling you that you need to change all the electronics. Today these basses are sought after in original condition and yours seems to be very close, except of the changed knobs.
IF the elecronic unit is gone (not likely to happen) it is possible to buy new units from a Finnish enthusiast. He Selling the Electronics on ebay. IF the pickup is gone it is possible to buy from Another Finnish guy who winding his own true copy of the pickup, or to buy from Kent Armstrong or Aron Armstrong (I cannot remember who is located in UK, but it is father and son, one located in UK and one in USA). The pickups is the weak Point of the basses as the originals tends to die very abrupt, and they are filled with epoxi, i.e. can't be repaired. My feeling is that this is more likely to happen on the somewhat newer basses, from 1983 and on. The pots are of good quality. Get anybody with experience of Electronics to Clean the pots and maybe to change the Contacts for the battery and you will probably have a bass for your Life. I have had 8 SB-basses during the years but the Three I have now will stay.
Btw, there are no copies of these basses, only newer releases. But this is the original one, fore sure. And it is one of the earliest. The headstock on the bass was changed between 80 and 81. This is called the bat-ear headstock.