Hi PJS,
As you are not changing from Cash Basis to Accruals Basis the Transitional rules and guidance referred too would not apply.
Please refer to PM163090 & PM163160 which covers the continuance of a notional trade, which appears to apply in this case.
Whilst we understand the impact outlined in your query, we would suggest that it would be between the individual members of the partnership to take this event into consideration when agreeing the shares of profit/loss arising in the first years accounts of the partnership.
We cannot find anything else that would suggest a different process for declaring the income received by the sole trader under the Cash Basis regime.
PM163090 - Commencement and cessation
Thank you.