Metamodelling and model transformation play important roles in model-driven engineering as they can be used to define domain-specific modelling languages. During the modelling phase, modellers encode domain knowledge into models which may include both structural and behavioral aspects of a system. The contribution of this paper is a new web-based metamodelling and model transformation tool called WebDPF based on the Diagram Predicate Framework (DPF). WebDPF supports multilevel diagrammatic metamodelling and specification of model constraints, and it supports diagrammatic development and analysis of model transformation systems. We show how the support for model transformation systems in WebDPF can be exploited to (i) support auto-completion of partial models thereby enhancing modelling efficiency, and (ii) provide execution semantics for workflow models. Furthermore, we illustrate how WebDPF incorporates a scalable model navigation facility designed to enable users to inspect and query large models.