Srilanka Railways 

This site is a scam, booked a train and paid for it, received confirmation of payment. Approx half an hour later get an email from them saying that the selected journey is unavailable, and suggesting alternatives, which are for different dates, and different trips (eg going to a different location). My originally selected journey is still available on their website for booking... Have requested a refund and will update following that.

This is NOT the official Sri Lanka Railways website — they admit it themselves in the footer, describing themselves as an independent travel agency. That alone isn't a dealbreaker, since I know the official Sri Lanka Railways site can be frustrating to register on and often throws errors. But that doesn't make this site a good alternative.
The real problem is the price. They sell 1st class tickets for around €32, while the actual fare through Sri Lanka Railways is a little over 1,200 LKR (roughly €3). That's close to a 900% markup. I checked other resellers too — most charge at most 2-3 times the official price, and some even sell at the same price as the railway itself. This site is a clear outlier on price.
If you're struggling with the official site's registration issues, look for one of the other resellers charging closer to the real fare — you'll save a significant amount of money.

Scam. Taking so much extra than the price through the real website.
Usually im good at spotting scammers but their website looks legit!!!

THEY ARE SCAMMERS. DO NOT USE.
They are not the official train company. Use railway.gov.lk even though it seems less intuitive.