Hi
@glenjarvis & firstly

to Cycle Chat from The Peoples Republic of Mancunia.
Some good points are raised here, not all I am in agreement with though.
Firstly, every post (including mine) will be personal opinion & eventually you will take from it the best parts that suit you at this time.
The Carrera TdF is a fine bike for it's price but was made to hit a price point as an entry level bike & to sell lots of units to newbies (we were all newbies once).
I agree with upgrading bikes generally, but in the middle to high end price bracket.
You might need to spend £200 on wheels, £80 > £100 on a Chainset/Crankset £25 > £30 on a derailleur & £15 on a cassette, as changing one part can sometimes necessitate changing another (long cage derailleur for a wider cassette) for example.
All of the above might save 250 > 300 grammes & give you a wider range of gears & a little better acceleration (wheels) also making climbing easier.
If your current TdF has a compact 50/34 chainset you will save a little outlay there. The numbers might be printed on the outside of each ring or on the spider of your front rings, if not you can simply count them.
I believe that you have 2 options.
1. Keep your TdF for another year, (change the saddle if you like) but nothing else, I don't think you would see much return on your £££ when you sell, (& you will sell).
2. Sell the TdF & get the best price you can for it, then buy the best bike you can manage to afford.
This doesn't mean a £1000 carbon racer.
A 2014 Giant Defy 3 (9 speed Shimano Sora) can be had for £449 from
www.rutlandcycling.com or
www.pedalon.co.uk
A Felt F 95 for £395 (9 speed Shimano Sora) or F 75 (10 speed Shimano 105) for £599 from
www.merlincycles.com
All of the above will be significantly lighter than your TdF, have lighter higher quality frames, lighter carbon forks, lighter higher end gears/groupsets, better range of gearing & out perform your TdF. (you might not even need to change the saddle).
I think you will notice the difference.
If you could manage to get £200 for your TdF you would need another £200 > £250 to get one of the above.
or you could spend £250 "improving" your TdF, (how much of that would you get back come sale time) ?
Best of Luck with your decisions/decisions (tough job this cycling lark)