Steve,
I have exactly the same problem, one very determined cat, every day, every night constantly in the garden going after the birds. You need to remove all ground feeders for starters. In the situation where your birds are also vulnerable to Sprawk attacks, I'd consider moving some of the hanging feeders too. I've had to cut back lots of vegetation, including all border plants to remove any hiding places.
Unfortunately the cat I've got seems to be a feral cat and I've never had such a problem as recently. Managed to save a Wood Pigeon the other day as the cat pounced on it but if I hadn't have been there it would be dead! This cat is a killing machine and probably stone deaf since I keep finding it sitting in front of the cat sonar monitors!
You could try getting one of the covered feeders and setting it up in the middle of the garden away from any shrubs/trees/fences or you could try window feeders/hanging feeders from hanging plant brackets on a wall of the house.
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Either way, the Sparrowhawk is likely to take far less birds than the cat, so siting feeders in safe positions from the point of view of cats should be your priority - ie nearer the house (so you can reach cat with bucket of water before it can run off!) and well away from any fences or shrubs where a cat can pounce from - or don't you have any trees you can hang the feeders from? - I've found these to be the safest providing there's clear fly away space all round them and the birds have sight of terrain all round.