Following the link from Tore and inspecting the ProtocolMessage properties, I managed to manually set the redirect url.

I've added this under .AddOpenIdConnect:

config.Events.OnRedirectToIdentityProvider = context =>
{
    context.ProtocolMessage.RedirectUri = "https://TestWebsite.co.uk/ids/signin-oidc";
    context.ProtocolMessage.IssuerAddress = "https://TestWebsite.co.uk/ids/connect/authorize";
    return Task.CompletedTask;
};