To add with what has been said already, I wonder if local events, have any impact?
South Tyneside is on the watch list, Fencehouses has seen a huge upsurge according to the Sunderland Echo.

Assuming S Tyneside locks down and 300 people are self-isolating in Fencehouses, then it could have a major impact on services.
Do you stick a smaller Solo on the 4 to cope with any drop in numbers and allow greater distancing on the 81 etc across Washington by using Mercs?