This appears to be a salutary lesson on the use of ‘anchored’ surfaces at a heliport. The safety assessment, prior to the decision to fly the CV22B to Addenbrookes, might have picked up the fact that the surface was inadequate for the type.
From ICAO Annex 14, Volume II:
3.1.1 A FATO shall:
a) provide:
1) an area free of obstacles, except for essential objects which because of their function are located on it, and of sufficient size and shape to ensure containment of every part of the design helicopter in the final phase of approach and commencement of take-off in accordance with the intended procedures;
Note.— Essential objects are visual aids (e.g. lighting) or others (e.g. firefighting systems) necessary for safety purposes. For further requirements regarding penetration of a FATO by essential objects, see 3.1.4.
2)
when solid, a surface which is resistant to the effects of rotor downwash; and
i) when collocated with a TLOF, is contiguous and flush with the TLOF, has bearing strength capable of withstanding the intended loads and ensures effective drainage; or
ii) when not collocated with a TLOF, is free of hazards should a forced landing be required;
Note.— Resistant implies that effects from rotor downwash neither cause a degradation of the surface nor result in flying debris.