A quick look at Farnell, 47uf @ 35v is only a few pence, but you can lose the zener for a resistor. In fact then you could lower the capacitor still further, does it matter how much the voltage rises to ? Higher voltage electrolytics tend to store more energy for the same size. As well as cost there are still pros and cons to both, not least the lifetime of electrolytics. Its up to the Mike to weigh them up, but maybe he prefers silicon ? :)
Colin =^.^=