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With regard to why its £120 and not £60, this is just down the the manufactures knowing there are not that many unvented systems on the market and with little demand for elements they have hiked up the price, over time as unvented systems get older and more elements are required the price will come down.
I should however point out that the £30 one you have seen else where is probably for a vented emersion, and can not be used, since vented ones are not designed to work at mains pressure, as such the unvented elements are constructed to a higher quality and can not be substituted.
I would advise against doing any work to the emersion yourself as you would require building control approval, since it falls into the same category as electrical work which needs a qualified person to commission the unvented emersion, and could invalidate your house insurance should anything go wrong, due to a number of DIY attempts resulting in emersion tanks blowing up.
Please let me know if I can be of any further assistance.