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I have a rattle from the driver side (RH) cylinder head. It's mainly when decelerating or when the engine is struggling a little. I was hoping it would go away but if anything it seems even worse. Any ideas what may cause this.

You might look at the cat on that side. They can break up and or loose and rattle in the exhaust pipe. If you are hearing a rattle from the cylinder head itself, something is very wrong in the valvetrain, most likely.

Have you tried to recreate the sound when standing close to engine to pin down the sound origin?

The cats have been completely removed on both sides so it's not that. I did remove the exhaust pipe and found the tiniest bit of metal that would no doubt have rattled but it didn't solve the problem. Yes I have tried to pin it down and am 99% sure it's originating from the cylinder head. It starts up without any problem and hasn't affected it's performance at all.

The engine has hydraulic tappets, they can fail to "pump up" on start to close the clearance between the rocker arm and valve. They usually pump up and quieten down after a few seconds running but I suppose they can fail completely and rattle. Maybe worth running for a short time at idle with the rocker cover off to see if that is where the problem is?

That is good advice as I was wondering if that was possible. I presume that by running it for a couple of minutes and being careful to make sure it sits ok on the seal when putting back together all should be fine?

I have run the engine with the rocker covers off for a short time, after fitting roller rockers to check all was working well. Keep an eye on the oil dripping off the rockers onto the hot exhaust!

If you have the later "plastic" type rocker gaskets they can be re-used several times without any sealing problems. The early gaskets are paper and tend to fall apart when disturbed. It's not easy to see what type you have until you remove the covers, although you can just see the edge in the joint above the exhaust, the later type are about 1mm thick and paper much thinner. Phil Bleazey did replacement rocker cover gaskets in paper I think. Not sure if the S&S later type gaskets are still available or not, they were not cheap I remember at about £35 each a few years ago. Not tried it in this position but silicone gasket sealant works well usually.

I understand MMC have produced full gasket sets for the S&S. I'm not sure if they supply them individually or if you have to buy the whole engine set.

Update - removed the rocker cover and all seemed fine ie no rattles. Everything was moving as it should so lightly revved the engine and still no rattles. Put all back together and drove down the road and the rattle persists. Removed the mudguard in case it was that and still rattled. I have now resigned myself to having to put up with it to see if it just goes away.

Suggest using a rubber mallet and tapping all around the area where you think rattle is coming from. For me I discovered a rattle of mine which I originally thought was engine-related actually came from within headlight.

From personal experience I’d look at
A) urban cooling fan blade loose or fan housing rattling against the engine.
B) the headlight bowls cracking at the base and rattling.