No hot water on all taps with unvented system

what make of unvented cylinder ?, if megaflow internal air bubble may need repleneshing ,if other make external expansion vessel could be at fault,it could also be filter in mono block valve ( the prv /pressure reducing inlet valve ).it may still be 3 bar after prv but have you had filter in mono block valve checked,to replenish internal air bubble turn water off run hot water taps till nothing comes out,turn water back on leave one hot outlet open till water comes out.then close outlet.but it sounds like debris in your filter.you only need to drain it a little a 2 gallon bucket will do,use drain off cock with short hose,but you should have filter checked first,these cylinders require an annual safety check by a g3 qualified plumber who will be able to rectify any problems.