Don't bury your head in the sand. HMRC won't forget you or the money you owe them.
Having said that, do make sure that the money they're asking you to pay is correct. HMRC aren't perfect! If the debt includes penalties, can you appeal against them?
If you do owe the money you should contact HMRC as soon as possible. The sooner you do it, the more sympathetic and helpful they'll be.
get more time to pay
pay your bill in instalments by direct debit
And, if all this fails (it shouldn't) or you just want some advice contact us on 07866 800721.