Hybrids are generally suited to the things which diesels aren't and vice versa. A diesel will sit quite happily all day long at 70 mph on the motorway and you will get very good fuel consumption from your long motorway journey. The typical hybrid will have very little opportunity to run on its electric motor and so will be unable to recycle hardly any of the energy gained on the throttle over-run or light braking. Therefore your 46mpg represents the fuel consumption of your petrol engine with very little of the hybrid system input. In town driving, however, the diesel fuel consumption will rise dramatically whilst all of the hybrids electronics will be happily running you car on harvested electrical charge from the last braking phase or downhill coasting and the electronics will bring the engine online just for short burst acceleration or battery augmentation. Hence, around town the hybrid fuel consumption improves on your motorway figures whilst your diesel seems to use fuel like there's a hole in the tank. The hybrid performance comes into play very low down the rev range and therefore at very low speeds such as pulling away from traffic lights or away from junctions.. The diesel has a lot of torque but it is mostly towards the lower middle of the rev range and therefore gives the diesel engine car a lot more punch at motorway speeds.
My fuel consumption always drops away on fast A roads and motorways. I could easily outperform my 11 plate 1.8 Lexus CT with my 1999 T plated Volvo S80 2.5 litre diesel on both power and fuel consumption on the motorways. However, the old Volvo would be left standing at the traffic lights by the CT. The CT would be gone in an instance whilst the S80 would still be mulling it over as to whether to spin up its turbo-charger.