Large multi-dowel connections can provide the strong and ductile connections required for large, highly-loaded timber structures, but their slip under load is not well understood. This is important because accumulated local displacements at connections can have significant implications for overall building serviceability. Empirical relationships for the slip of a single-dowel connection do not capture the dowel interaction effects of the multi-dowel connections used in larger structures. We present the results of an experimental test series and probabilistic numerical analysis investigating the development of stiffness in multi-dowel timber flitch plate connections. The influence of the diameter and number of dowels on the stiffness of the connection are investigated, including the influence of off-centring of dowels due to manufacturing tolerances. The test series is used to validate a probabilistic model for the stiffness of such a connection. The model incorporates the nonlinear stiffness and hole opening observed in single-dowel connections to predict the behaviour of the group. The study shows that the random off-centring of dowels in multi-dowel connections reduces the range of displacements over which the connection displays zero stiffness, but that this zone is not eliminated as a result of irreversible hole opening under load.