Post in case this assists anyone.
It appears the current policy on escalating an application is either – been waiting over four weeks or booked travel prior to lock down (and can prove it). It is worth noting that it was the advice of HMPO not to apply during lock down.
I fell in to the latter and once escalated moved from ‘documents received’ to application approved in two days.
A further top tip – try calling 01633 285422 rather than the standard number as they actually answer rather than being placed on automatic call back.
Finally don’t bother emailing to raise things as it seems they just send out an unhelpful generic response.