Plot thickens! I have done a bit more investigation and peered down the hole in the sump unit that feeds the top spray arm. There appears to be a shutter valve arrangement. I fiddled a little bit to see if it moved - it did a little. Then I put the fittings back in and switched on the machine. Amazingly the top spray arm rotated
but now the bottom one didn't
. Oh well that rules out a problem with the top spray arm. Then I reset the wash cycle and tried again - now the top spray arm stopped working and the bottom one worked again - now I am really confused!!
Had a look on the Bosch parts website (https://www.bosch-eshop.com/eshop(bD1lbiZjPTAwOQ==)/bosch/gb/prodp.htm?prod=SGS53A02GB%2f38) and this shutter valve appears to be part of the water heater - refered to as an alternating water distributer. Not sure if this is faulty or whether it is a problem with the controller. may do a bit more investigation but may just give up and get a new machine - not worth the cost of repair.... unless anyone out ther knows differently!!!