Re: DOT 5.1 is it worth the extra £ ?
DOT 5 is silicone based and cannot be mixed with DOT 3, DOT 4 or DOT 5.1. It is also not compatible with some brake system seals (no idea if the seals in any particular Alfa are compatible with it).
While DOT 3, 4 and 5.1 absorb water like crazy, DOT 5 does not. This has some plus points (you can avoid changing the fluid for far longer) it also has the problem that once water is in the system it is very difficult to get it out (and the water is likely to just pool somewhere in the system).
The other issue with DOT 5 fluid is that it tends to be slightly compressable, and can result in a slightly softer pedal feel.
Sometimes DOT 5.1 is badged DOT 5 on the bottle, but if you do buy one like that then make VERY sure it is DOT 5.1.
DOT 5 these days mainly seems to be sold for use in rarely used classic cars (taking advantage of it not absorbing water and thus long change intervals). Only vehicles I know of that use it as standard are Harleys.
All the best