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.
Businesses also sign 13 year agreement for motor and home insurance distribution.
First international acquisition for the firm.
Aviva's mid-market push comes under the microscope along with Brexit.
Purchase of West Midlands-based broker is County’s first deal of 2019.
Verlingue explains the history of the family-owned French firm and shares his thoughts on buying during Brexit uncertainty, organic growth, servicing clients and heli-skiing.
MD David Beswick says that the Broker Network-owned business wants to develop six hubs in total as it eyes £90m GWP by the end of 2020.
Deal is Saffron's third since it was bought by Broker Network a year ago.
First deal for Buckinghamshire-based broker since 2010.