My suggestion is IGNORE anything and EVERYTHING Halfrauds told you.......then start from the beginning.
The above statement can be used for any situation in which
Halfords have given advice about cycle repairs......
(you may have been lucky and spoken to someone who actually knew what they were talking about - but it is about as likely as it would have been if you went to your local butchers!)
As the above sounds a little defamatory I think I should be fairer to Halfords...... They sell some very good bikes at very good prices (They also sell some absolute crap bikes) Some of their staff are undoubtedly very knowledgeable and competent - but they have not yet managed to ensure consistency across their stores and some of their staff shouldn't be let loose with a spanner. I have actually seen a Carrera MTB supplied from my local Halfords with the front fork on backwards!
What do you recognise as being wrong with how the Subway works ....ie. do the wheels turn round? Do the gears work?
If for example, the whole drivetrain is worn - but working ok, you have nothing to lose by just continuing to use it until it stops working ok.
Start with the important things.... what's the worst thing about how the bike works or doesn't work?
Wheels - if they revolve, good - that's a start! If they spin - but not smoothly, strip the spindles down and re-grease (and replace the bearings if needed. This will cost you about £4 if you do it yourself and need bearings - but you will also need a pair of cone spanners to undo and adjust the axle nuts....so that will be another £3-4.
If the wheels are buckled get yourself a spoke key - and read Mickle's post on how to true a wheel - cost about £2 for a cheap spoke nipple key.
Cables - do they work? If so, but not smoothly pull them out - oil them and put them back. If they're seized bin them and go to Wilkinsons and buy new ones (Not the best quality - but they will do the job and cost about £8 in total for your brake and gear cables) While you're there if you need new brake blocks they sell them for about £1 - and they will do an adequate job of stopping your bike.
Now to that drivetrain...... slip the chain off the smallest ring on the crank and hold it clear of the chainwheel. Now spin the crank.....if it spins freels great! If not, it will need stripping down and maybe a new bottom bracket/cartridge installing (I don't know what crank your bike has).......but it may just need cleaning and re-greasing...... This is the hardest job so far and does require some expenditure as you will need the correct tool to pull the cranks off the spindle and then to remove the bottom bracket...
So at this point.....(and maybe I should have pointed you towards this sooner) is it worth buying a cheap but basic bicycle tool kit with the tools you will need to tackle the jobs required?
A basic tool kit like this one....
http://www.ebay.co.uk/itm/Bicycle-B..._Cycling_Tools_RepairKits&hash=item2a2472fb88
Will cost you £30 and will be perfectly adequate for a year or 2 and over time you can upgrade tools as required.
Does it steer ok? It may need the headset stripping down if not.
As you get to each job - seek advice on here (search threads first as it is likely that others have trod the same path before you)
and also search through the videos on Youtube - there is a video on there for almost any cycle maintenance task you can think of.
Don't be put off - keep it cheap and cheerful and tackle the jobs step by step. You will learn loads, your confidence will grow as will your ability and you will have the satisfaction of having kept an old faithful in service....
You will also learn how to maintain any future cycles you get to stop them getting into the state of your Subway.