Bosch won't drain on programmes

[I800dave]

My Bosch excel 7 washing machine is flagging up E18 which is supposedly a blocked pump, I've made sure it's clear at the impeller end/filter and the sink end, it won't drain on programmes but WILL on the drain only function! My thoughts with reading this forum over the last few months is possibly a water sensor? Weight or flow or worse case the pcb? 

We have cover for it with an extended cover but I'll get charged if no fault is found so want to fix it myself really, the machine is almost 5 years old, thanks for reading