I have the same issue, although mine has always been in the ground outside. In my case, I suspect some kind of pest gets to it as it also gets nibbled, although that may not be the only issue. I suppose evergreens are always going to lose leaves periodically anyway, though, and it's going to look far more noticeable when it has such large leaves. It might help to pinch out the new leaves if you want it to return to a bushier look.

Individual leaves have a finite lifetime and even on evergreens they must eventually die and fall off. As long as it is the bottom leaves i.e. the oldest which are dropping this is perfectly normal. As lori suggests, if you want a bushier plant, pinch out the growing tips. I have the hybrid Fatshedera lizei growing up the back of my house and it is currently dropping many yellow leaves. I rescued it as a half dead pot plant from a bin at work and it is now about 2 metres tall.

"I have noticed that older fatsias in garden centres tend to be a bit yellow"

Probably from being in too bright sunlight - it is a plant that prefers a fairly shady site.

"Since I repotted it"

I'd be inclined to plant it outdoors. It is fully hardy in most of Britain.

Resin

Thank you for your tips.
I think I may have overwatered it somewhat because the yellowing of the leaves has slowed since I stopped watering so frequently.
It is in the shade for most of the day. I wish I could plant it in the ground but the only area shady enough is a paved patio where it looks good with ferns and spotted laurel.

Yup, mine looks yellow and tatty most of the time, too. I have heard they tolerate being cut right down. Is this true? I was vaguely thinking of moving mine soon - do they object to this treatment?

Sure they'll object (what wouldn't!!), but they'll tolerate it and re-grow OK.

Resin

I repotted my fatsia japonica this year and have exactly the same thing happening....it's normal. Just pinch out the growing tips and it'll bush out again. The frost got at mine year before last and nipped out the growing tips and it soon sorted itself out....lots of new lovely growth from the bottom in no time.