CGT disposal on grant of long leases between connected co-freeholders if beneficial interests remain largely unchanged?

Is there a 'disposal' for CGT purposes?

The CGT handbook provides a disposal usually means parting with beneficial ownership but does not define disposal. Does anyone know an authority on whether this is a disposal?

Background

A & B (siblings) and C (B's wife) are freeholders & joint tenants in common of an inherited residential property in England. A lives in the property so benefits from PRR regardless. Benficial ownership is governed by a declaration of trust.

Details of the trust

The trust was set up as a mechanism for protecting existing interests following the renovation, funded soley by B&C (the non-residents) ensuring they benefit from increased value provided A's interest did not decrease in value.

i.e. if, following renovation, Flat 1 did not match A's initial interest, and Flats 2&3 were valued more that B&C's initial interest, A would be entitled to a portion of this to make up any deficit (protection for an unlikely event).

Following the works and a valuation, beneficial interests have crystallised as:

A: beneficial owner of Flat 1, valued with a small uplift from A's initial interest.

B&C: beneficial owners of Flats 2&3

The trust also sets out that long leases should to be granted by the parties to reflect the post-valuation position.

Issue

Parties are looking to formalise the arrangement through by granting leaseholds (999 years) which reflect their respective beneficial interets i.e. lease to A for Flat 1, leases to B&C for Flats 2 & 3.

Is specialist tax advice necessary here? If so where would be the best place to look for this? Our property solicitor and accountant have been unhelpful and costly in time and money.

Thank you very much in advance for any guidance.