Deal is the consolidator's second in 2019.
Purchase has been approved by the European Commission.
Newport-based broker says it aims to keep the Bayliss & Cooke brand, with all 14 staff remaining with the firm.
Broker Network Partners rebrands as Ethos to separate deals from its networks as CEO Des O’Connor reiterates that the networks will remain apart.
The long-rumoured deal was finally announced in January this year.
The deal is set to complete in June and is Adler’s fourth in three years.
The deal is described as a “merger of equals”.
The businesses emphasised that the talks were in a preliminary stage.
Reports suggest the two brokers have held preliminary talks about a potential deal.
Castle specialises in tour operators, food and leisure, and health and beauty, and will act as a hub for the Lincolnshire region.
Aon bought Henderson Insurance Brokers in October 2017.
But broker reveals pre-tax profit fall due to transaction costs related to $5.6bn takeover by Marsh & McLennan Companies.
Several sources have indicated that the business is on the market.
Price published in the company's financial results for 2018.
Business to expand into more countries in Europe after buying Optis Insurance in Ireland.
Hertfordshire-based broker specialises in equine insurance and is GRP's second deal in 2019.
Provider proposes to cut 711 roles across Europe as integration process continues.
More deals in the pipeline as Irish regulator approves purchase of Dublin-based Robertson Low.
Business is made up of nine firms, one of which will be GRP's retail hub for North London and Essex.
The deal takes Greenwood Moreland to £10m of GWP annually.
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Business takes on £70m of Co-op’s debt in first buy since being bought by Qatar Re.
Its deal with TopMark Group follows its acquisition of Direct Group’s claims function from Ardonagh Group.