Lethbridge-Stewart: The Havoc Files [vol 1], ed. ?Shaun Russell
Second paragraph of third story (“The Ambush”, by Andy Frankham-Allen):
The colonel looked back at his squad. They’d made it as far as Charing Cross Station before the fungus stuff blocked their way. Up until now it had been safer to travel on foot through the Underground tunnels, but the Yeti had started advancing there too. Spreading out that web of theirs. He glanced at the civilian they’d rescued. The one man who could help them in this fight. Professor Travers.
This is a set of eight short stories in Andy Frankham Allen's Lethbridge-Stewart universe, looking not only at the gap between The Web of Fear and The Invasion, but also at the earlier and later life of the Brigadier. Four of the eight stories are by Andy Frankham-Allan; two of the other four are by "Dave Cross", who has no other Who of sf credits and may be a pseudonym. The two standout stories for me are the first, "The Enfolded Time" by Frankham-Allan, which actually resolved the UNIT dating controversy as far as that can ever be done, and the fourth, "Legacies", by "Norma Ashley" (definitely a pseudonym) which takes a sort of Turn Left approach to The Web of Fear. I realise now that these stories are all set after the first series of Lethbridge-Stewart novels but before the second, which I have just finished reading, so perhaps I should go for the second Havoc Files volume next. It's fairly continuity-laden, but you can get it here.