This is the one that is a copy of the Joie Steadi. Mothercare had seats made by several different brands.
It's a good seat, especially if used rear facing. If it doesn't come with the instructions, you can download them here: https://uksupport.joiebaby.com/hc/en-us/articles/4408451854996-steadi-manual-and-specs
There is a risk inherent with buying second hand - you don't know if this seat has ever been in an accident, you don't know how long the person has had it and if it's ever been damaged or repaired. There is a possibility they may lie about these things or omit information when put on the spot, out of awkwardness, maybe having genuinely forgotten, assuming it's not important or out of wanting to get a sale.
Also, it's possible/likely that the seat will come without instructions, and may come with missing or damaged parts. Car seats must only be used with their original/official covers, inserts and accessories. If you're not familiar with the seat model it might be difficult to work out on the spot whether all the parts are there and the right bits. It might be worth looking at some videos on the seat e.g. unboxing or reviews or how to use, so you can see what it's supposed to come with (Joie Steadi will get you better results here).
Using a car seat incorrectly can render it totally useless - it will only offer the advertised protection in a crash if it's properly installed and used. So it is important to get it right and that is easier to do if you have the instructions, all original parts etc.
If you do choose to buy second hand, inspect the seat thoroughly and don't use it if you can see any stress marks (white lines) in the plastic, or if the cover shows any signs of mould. The polystyrene should also be intact and you should ask the seller about the history of the seat including when they bought it. If they aren't sure because they originally got it from someone else, that adds another layer of mystery. Don't use a car seat which is over about 10 years old, if it's a very cheap brand (like Nania or most generics) I'd lower that to 5-7. Check all the moving parts like straps, recline and headrest are moving smoothly, and if any straps come out to change the height, mode, etc (I think not, in this seat) check they have been routed the right way through the seat and not twisted or damaged at any point along the length of strap.
If you want a budget friendly option to buy new you could look at these:
Joie Juva - £45, suitable from newborn up to around 18 months, compatible with a Joie pram (Mirus or Muze)
Cozy n Safe Fitzroy is about £65 and does RF up to 13kg (around age 2), FF up to 18kg (around age 4). Not the most amazing seat ever, but similar to the Joie/Graco ones, and cheaper. I do find this tricky to use and think it has a medium-to-high chance of accidental misuse, but it's a very good price.
Or Halfords have one called Impala which is £60, rear facing up to around age 4, and meets the new safety regulation. Absolutely no clue of the performance of this one, it does look fairly flimsy, but the fact it's rear facing is positive. Would caution again, may be difficult to use correctly.
If you wanted to get the updated version of the Mothercare Boston, it's currently for sale branded by Graco, called Graco Extend R129 and costs £90.