Yeah britpart ones work well.. the seal.. well just use silicon to hold the seal down the attach the plastic part afterwards to it.. did one recently with my galv heater box..
I'm ordering one from westernpowersteering..
I've shopped around and for the effort of the extra £100, these guys have a good rep and looks like i can fit it and trust it.
Few of the ebay adverts i've been are cheap seal kit replacements etc