Large (36mm Diameter) 40 Gram Coin, Gold (Id: 1902, 5 Pounds From Great Britain)

jmac, I really hope that is what you have! I used Google reverse image search to identify it, and there were MANY of these with descriptions in Chinese.

OP's original post suggests that not much is known about it by the OP, hence the need to find out more about it.
That's OK.
The questions that arise in my mind from this, is to ask:
Does the owner have a record of previous ownership? (provenance)

The 'Coincraft's 1998 Catalogue of English and UK Coins 1066 to Date' ( page 420), warns of the
"widespread existence of forgeries, known euphemistically as restrikes, or as being 'wrong'. Many of these are of the correct weight, and they often mislead experienced dealers.
.........
A genuine coin will often have an 'old' look, frequently with minor surface marks, whilst a forgery often has a 'clean' look, and a somewhat weak strike".

Although the above mentioned comment applies to gold Five Pound coins dated 1887 and 1893, I see strong reason why it should also apply to gold Five Pound coins dated 1902.
-Hence the need for having a record of previous ownership.

Considering the potential value of this piece, XRF testing at at bullion dealers' premises would be strongly justified. A fee for this service would most probably be charged.

In the meantime, the coin that is the subject of this thread should be compared with the best possible quality pictures that can be found on
Google Images:
'1902 British Five Pound gold coin'.

Sorry for seeming to be excessively cautious, but it has always been my inclination to check the authenticity of any coin in this potential value range.