Five years for electric shower is enough but sometimes is good to check electrical connection due to high current If electrical connection in good condition i advise to buy new one . Spare parts expensive and no guarantee what will happen with others parts .
Just buy a new one, I bought a Triton 7.5kw shower this morning for £ 45.00. Don't forget you should have an RCD on your fuse board/consumer unit and it should be installed by a competent person.
I had replaced an older Mira sport ( approximately 5 years like yours ) with another Mira sport, it looks a little different but all the connections I.e water inlet and cables, were in the same place as the older model so replacing was very easy and took little time to replace so labour should not be that expensive. I had looked into it before replacing and found out that it was best to replace than to repair.
Spare parts are readily available for Mira showers but from personal experience the price is quite high making it difficult for a trades person to repair at a price that is considerably cheaper than buying a new item. At 5 years old you may decide that the extra money is worth paying to have a new product. If you were to buy a direct replacement then the cost of fitting may be kept to a minimum if there is no requirement to alter the pipework or electrical supply.
I would suggest to get new unit which will have quarantee for at least 2 years. It depend which one you will choose
It will save you a money as replacement parts could cost up to 90% of new shower
I concur with Stuarts earlier feedback. As an alternative option, depending on your budget and plumbing arrangement in your property, you may want to consider changing your Electric Shower, from one fed by your gas boiler. If you consider using this as an alternative, it will generally be around 70% cheaper to use than one fed by a gas combi boiler and you generally get a much better flow rate. In terms of cost, it obviously depends on your hot water demand.
I would advise you to have a Qualified Electrician look at it first, to check there is power(Both Live and Neutral) present.
Also have him check the cable size before you buy a new one, or you may find you have to upgrade the cable.
The Electrician should be able to advise you on the max Kw (size/power of the shower) you can have with the existing cable.
and ensure you have an RCD protecting the shower circuit.
Ensure you are given a minor works certificate from the Electrician carrying out the work.
Hope this helps