My daughters advent laptop will no longer charge/operate when plugged in, she has tried a new charger, but this doesn't rectify. I suspect it is a problem with the DC jack, is this a relitavley cheap repair?

It generally is a cheap repair, however it may be the battery. Quite frequently when that happens the battery is mainly the problem. The DC jack does not get affected unless it is disturbed.

The first thing you should try, as you have done, is using another charger, perhaps you have a spare one lying around or know somebody who has an Advent laptop. Each manufacturer generally uses the same charger for their laptops, though of course they will update them from time to time.

If the problem is with the DC jack then it won't be a cheap fix as the jack isn't a modular component like the memory, HDD, battery or the opitcal disk drive. It will affixed be to the laptop's motherboard and not readily accesible, even to a laptop techncian.

If you have another laptop that uses the exact same battery (you would need to check the battery and laptop specifications) I suppose you could use the good laptop to charge batteries for the damaged laptop, however this clearly isn't an ideal solution.

If your warranty has expired, then short of replacing the laptop (You can get some quite decent second hand laptops of Ebay), your best bet may be taking it into a repair shop. Be careful that they don't rip you off though. Ask them if they can or can't definetly fix it and how much will this cost, if can't give a solid figure it probably isn't worth it.