Need to change the brake fluid in my Hope Mini's, now can I use bog standard Dot 4 or 5.1 fluid from Halfrauds at £12 a litre or do I need a "bike" specific fluid for £6 a thimbleful?

Any 4 or 5.1 you fancy. There's no such thing as car/bike/bicycle-specific. If you're bothered at all about getting higher performance fluid, go to a motorbike store, where one tends to get high BP fluid cheapest.