Would This Be A Good Buy?

It is an extremely common AE3 type of Valens. See type 6 on this site:
http://esty.ancients.info/ricix/

They were not surface silvered. The silvery look must be an artifact of the photography. The flan chip at 11:00 is a big negative. Long ago I saw cigar boxes full of AE3s of this period -- thousands of them. It is one of the most common Roman types of all. It might be worth $5 but one without the flan flaw would not cost much more. A super EF well centered and bold coud be a lot more, maybe $40. But, in my opinion, this coin is not worth anywhere near £20.

It does not look like a Siliqua, not the reverse design anyway.
The tone of the photo, the dark grey, gives the impression of it being silver.
I reckon it is an AE (copper alloy). Certainly, as Gil-galad states, the workshop insignias may make it rare, but for even £10 you could buy a reasonable AR Denarius.

Siscia is THE MOST COMMON mint of the "SECVRITAS REIPVBLICAE" type for Valens and his brother Valentinian I.
It was only an AE issue.

However, those from the Rome mint (R● SECVNDA, R●TERTIA) seem to fetch a high price.
For example, the below photo is of a Valens "SECVRITAS REIPVBLICAE" I sold to a buyer from Italy last year by auction, it went for £25.

That is what I thought at the time as well!

Auctions are a minefield for us sellers, as coins we think will sell poorly, if at all, go for a higher than expected price, and then coins of quality, like a fine ancient Greek AE, go for around £1.

Below is a photo of the example I mean, an ancient Greek AE from Amphipolis, that only sold for £1.71 a couple of months ago! The bronze itself was worth more than that!