I have a Halfords Carrera Vengeance 2011/12 bike:
Carrera Vengeance Mountain Bike 2011/2012 The Carrera Vengeance Mountain Bike will have you powering up steep hills and across rugged trails in no time. It has a Suntour XCM-V3 suspension fork which soaks up bumps like a sponge, and its SRAM 24-speed gears will make the afore-mentioned hills a piece of cake.
Approximate Weight (KG): 14.6kg
Forks: Suntour XCM-V3
Frame Material: Alloy
Front Brake: Tektro IO Mechanical disc brake
Number of Gears: 24
Rear Brake: Tektro IO Mechanical disc brake
Wheel Size: 26"
Chainset: SRAM S200 3.0 chainset
Fork Lock-out: No
Fork travel: 100mm
Frame Colour: Satin Gunmetal
Front Mech: Shimano FD-M190
Gear Shifters: SRAM X4 trigger shifters
Handle Bars: Alloy oversize riser bars (30mm rise) (31.8mm diameter)
Headset: Semi integrated
Hubs: Alloy with Q/R
Pedals: Wellgo alloy platform
Quick Release Front Wheel: Yes
Quick Release Rear Wheel: Yes
Rear Mech: SRAM X4 rear mech
Rims: Double wall alloy rims
Seatpost: 350mm Alloy 31.6
Stem: Ahead aluminium stem
Tyres: Kenda Tyres
But the gears are quite bad IMHO. It takes a lot of force to switch gears. I'd also prefer bigger cranks as I pedal on roads a lot. I'm perfectly happy with the brakes and the fork.
I'm looking to buying a set online, e.g. from https://www.merlincycles.com/groupsets-81168/ , but they come in a million sizes. How do I find one that will fit my bike?