Though classed as a Government supporter in 1747, for reasons unknown Colebrooke changed sides, and when re-elected in 1754 at Maldon after a contest, was listed by Dupplin among the Opposition Whigs (as were his younger brothers James and George). But by 1756 he held a secret service pension of £600 p.a., of which the first half-yearly instalment was paid to him on 13 Feb. In July 1760 Colebrooke unsuccessfully applied to Newcastle to be appointed resident at Hamburg, and before the general election of 1761 offered Government ‘to bring in two Members at Maldon ... and go out himself if he could get a consulship, or any other employment to live abroad’.
The secret service pension was continued to him till he kissed hands for minister to the Swiss Cantons; there he ‘exhibited to a frugal people an unprecedented mode of splendour and profusion’.
