Hi, Sorry, just read the do's and don'ts and missed a few bits of info.
The tank is 63 litres.
There were two inhabitants Diplo - the Red Cap who died and Don the fantail who is thankfully still with me. Diplo was 3 inches or so long and Don is about 3 1?2 inches long.
I use a fluval 3 filter system.
Water changes every week of 15 - 20% and a monthly change of 50%.
I use a standard King Britiish water dechlorinator with every change.
I test the water about every two weeks or so and the reading are always well within breed toleraces.

Sounds like he may have had some sort of infection going on in his wen.  They can become infected and even get a fungus.
What size tank do you have these guys in?
When you say water conditions were fine exactly what were the results of each test and which tests did you do?
Orandas need to be kept in pristine water condtions or unfortunately they can get an infected wen.
They can live up to 30 years if cared for properly.
 I'm sorry you lost your fish.  2 years is young.  Most goldfish with wens don't get their full wen until they are 3-4 years of age.

Your nitrite and ammonia should always read zero, nitrates should be no more than 40 ppm., 20 ppm is even better!
Glad to hear you will get a bigger tank.
You know there is a difference in surviving and thriving.  You will see such a difference in your fish if they do not live in crowded condtions!  They will be gloriously beautiful and you will enjoy them so much more.

The thing I've learnt most, is that goldfish need a lot of space to grow and exercise. What cold water tanks have you kept in the past? Just the small 62 litre?

The guys here have pretty much hit the nail on the head. A bigger tank will be much appreciated by the fish!
For fancy goldfish, the minimum stocking is around 80 litres/20 gallons for the first fish and then count 40 litres/10 gallons for each additional fish there after. So depending on how many fish you'd like to keep, you can work out a rough tank size using the guide
I think tanks also need to be a minimum of at least 4 foot or so, to allow for exercise.

50% weekly water changes is also a good way to go along with having lots of filtration, generally 10x the tank size is advised, so for example in a 150 litre tank, you'd want around 1500 litres per hour being turned over.
If you shop around for external filters (these have loads more room for media than internals), you should be able to find a good deal
I know a company in the UK (All Ponds Solution) which sell reasonably priced filters -I currently use their 1400+ model- and I'm pretty sure the filters are available elsewhere under different names (SunSun, Eagles Aquatic, Aquatop).