Which dual immersion cylinder and heat retention radiators?

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I own a flat. There is no gas supply to the flat. It is fitted with an electric water boiler/cylinder. The boiler is getting long in the tooth and I've been thinking of replacing it with a more modern unit for some time now.

Last week I had an EPC inspection done. One of the inspector's recommendations to increase the flat's energy performance was to fit a new 'dual immersion cylinder'.

What should I buy to provide the best performing unit for my tenant?

He also mentions that some of the radiators could be replaced with 'high heat retention' storage heaters. Once again, could someone please point me in the direction of those?

The radiators are on an economy 7 tariff.

Thank you.

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Thanks for that. As it's old, I assume it's a single. There's a mix of rads. One is less than two years old. The rest far older.

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It's mains pressure fed. So, here's nothing too trick in replacing with a twin feed? I suppose I can't go wrong with Dimplex when it comes to rads.

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Thank you for your help.