However, if your existing seal is outwardly in good condition, and stays in place on the edge of the boot compartment opening, that's half the battle won.

In case you aren't aware, the water doesn't get in over the top of the seal, it gets in under the seal, because the original sealant which comes pre-injected into the underside of the rubber seal has hardened and cracked.

In this case, the cheaper option is to go to B&Q, and buy a tube of non setting butyl rain gutter sealant, then just get the seal off the car, and inject a new fillet of sealant into your existing seal, and simply refit it - that should be problem solved. the only other place it's likely to get in is around the tail light seals - which are a rotten design, but will respond well to the butyl gutter sealant.