Replaced the impeller jug and all is working perfectly. Deffo worth a go before you call someone out! I got mine here http://www.leedsappliances.co.uk/catalogue/bosch-424099-dishwasher-impeller-jug.html - was cheaper than espares and delivered quickly even with standard delivery.
My 6yr old Bosch Exxcel dishwasher's developed the same problem...I've replaced the inlet valve as everything else (water matrix, pressure chamber etc) seemed very clean - but the problem still persists. Should I change the impeller jug as well, or any other ideas?
Thanks!