Just wondered if anyone else has an Oranda Red cap fish, is it normal for them to occasionally sit on the bottom of the tank, or should they swim all the time ?

most goldies are constantly on the go, but if they are young then they will occassionally stop to rest.
please check your ammonia, nitrite, nitrate and ph and give stats on your set up. sitting on the bottom can also mean bacterial problems.

So

Ph: 7
Ammonia : 0
Nitrite : 0
Nitrate : 10

any good ?

yup great, but how often do you do water changes and how much? do you gravel vac as well? even though your stats are excellent bacteria can still build up. have you noticed anything else wrong, such as gasping or red streaks in the fins? if not then hes probably just resting.
is he on his own in the tank and what size is it? fantail types need 10 gallons and filtration should be 10x the tank volume.

I would actualy say 20 gallons is the minnimum tank size. Some fancy varieties, it's true, don't get that big but orandas aren't one of them. Some people still do reccomend 10 as the minnimum though. Obviously, both minnimums are for a sngle goldfish.

Another cause of this, BTW, may be swim bladder problems.

Yes. This is absolutly fine! I also have a red capped oranda. She stays at the bottom of the tank quite often, which usually means that they are sleeping =)