Do I need to use the same size rings when replacing chainset?

[the snail]

If the chainrings are worn out, then it's quite likely that the chain/sprockets are very worn (unless they've already been changed) if you run a very worn chain on new chainrings they will wear very quickly, so you really should change the chain and freewheel.

 

[Ajax Bay]

you really should change the chain and freewheel.

The Apollo Evade spec suggests the OP has a freehub and cassette (I may be wrong though). If it is a freewheel then I think one with an 11t sprocket will be hard to find.

 

[djmagnifique]

You haven't said why, which might help you/us.

The chain has suddenly started slipping when setting off and the chainset was the only place I originally noticed any wear.

So i'm guessing that i'm going to need to replace the cassette and chain then or will I need t replace the chainset as well?

 

[Ajax Bay]

i'm guessing that i'm going to need to replace the cassette and chain then or will I need t replace the chainset as well?

Given the age of the bike and the distance you've cycled it, the chainset is rather unlikely to need replacing on account of wear. If you want to, post a picture of the chainring teeth from the side and you may get an opinion from @Yellow Saddle. Alternatively the issue (of chainring wear) has been discussed and illustrated ad nauseam (no that's not fair) and some cursory searching will find you the threads. Just replace both the cassette and the chain and you will have a happy bike (I predict). With the tools (chain whip, cassette lockring tool (and spanner to fit) and chain link splitter (to take a few links out of the chain you buy to get it the correct length) this task can easily be completed in 10 minutes. The chain I've linked to, and most chains sold like this come with a quick link (SRAM call it a power link) to join the chain 'permanently'. In case you haven't read this already, Sheldon Brown's article is a fun read. Be sure that you have a freehub and cassette not a freewheel (link). I'd get the cassette I linked to earlier (an LBS will be unlikely to stock this but assume could get it in) and either fit it (but I have the tools) or ask the LBS to supply the chain and fit.
As an aside, while the chain is off, it's an excellent opportunity thoroughly to clean the front and rear derailleurs, particularly the jockey wheels and the cages. With a good clean of the chainset too, you have a clean, efficient drivechain and many 100s of miles of excellent cycling (with a bit of routine maintenance (eg oiling)).