Consumer issue - faulty phone & rights to refund

Your chances are good.

Apple provide a worldwide 2 years warranty on new iPhones regardless of what the shop may claim or want to fess up to. Often they pass you onto Apple for such issues anyway, partially because Apple actually provide excellent after care service on their products and partially because the retailers know that Apple surpass what they'll offer so they basically don't bother taking on returns.

Your issues here are that you aren't the original owner of the handset (Establishing 2 years of ownership from new may be harder here, especially if the former owner didn't register it.) and the that the handset may have water damage, which may do away with your warranty. However only an authorised Apple service centre can confirm same under Apple warranty repairs.

Granted this is not what is laid down within consumer law but most of the time it's speedier to do it this way; my past experience with Apple Aftercare is that they generally surpass what is expected of them and that you'll get back as good as what you had, if not better. Give Apple a shout and see what they say, but don't forget that you can go Small Claims if there's no joy and the retailer is being crappy.

The OP correct re Apple, they will have nothing to do with this device, its clearly a refurbished model and any warrenty is with the reseller OP dealt with. Its also worth pointing out continuous repair is permitted rather than offering a replacement.

My concern is primarily Liquid damage if confirmed. Generally liquid damage on any mobile phone is unrepairable and the internal metal components deteriorate gradually, temporary fixes are just that, temporary. Generally liquid damage voids any warrenty as does physical damage unless the repairs carried out by authorised repair centre, apple generally just replace phones but at a substantial cost.

The real challenge for OP is determining who is at fault if liquid damage confirmed and proving it. Personally, I believe a small claims court action would not succeed if retailer defends the action. I respect others opinions but from my own experience in the industry, Liquid Damage a real challenge to prove, disprove and its pointless spending any more money on this device.

nothing to do with the networks as its very likely the op got the phone as sim only (the networks don't sell refurbished). Networks simply provide the communications.

Mic issues are common enough with phones and should be easily repairable. The scc will want to know how the mic issue came about and they won't give you original value, nor current new (refurbished) price either as its been used for almost 2 years. You can get a refurbished iphone 7 for under €200 at present.

I'd question the water damage excuse - how would it have been water damaged if it was in a drawer and the i[hone 7 was splash and water resistant. Did they not seal it properly after the previous repair?

I'd be challenging the water damage and also say that the original issue has not still not been fixed and you'd like a permanent repair to which you are entitled to