Agreed, you need to seek professional advice. What I do know is that if you are reasonable, the tax people are usually open to a repayment plan.

Good luck with it.

First,  you need to establish whether our chums at hmrc are correct in their assessment. They are better than they used to be on simple matters but still have a staggering error rate.
2016 year is still "live" and you can just file an amended online return for 2015 until Tuesday next week.
Assuming you agree their assessment, the best thing is to agree a direct debit for a comfortably affordable amount. If they have a dd in place, they tick an important box, and worry about others instead.
If you pm me I can at least point you in the right direction

Many thanks Pinkie! On way out, put will PM you tomorrow. 

David
I know what you mean.  HMRC are staffed by the same mixed bag as any organisation, and many of them decent helpful people that I have dealt with recently.  But they are demoralised, overworked, suffering progressive staff shortages, and inadequate IT(1).  Although the Treasury Select committee recently told HMRC their proposals for "Making Tax Digital"  are f***ing mental (I paraphrase slightly) - the expectation is that they will go ahead regardless.  Their approach to consultation is to carry out extensive meetings with knowledgeable 3rd parties like my own ICAEW, and then completely ignore the feedback and go ahead anyway.

As individuals, they are (mostly) lovely people.  As an organisation they are near collapse and very much systemised robots.

As an example, I have a client (now) - sister of a school friend, made redundant at age 60 from her post as head of childrens social services , after a lifetime with the employer received a package which included early retirement and involved large payments by her local authority employer into her  government pension scheme, which resulted in HER exceeding her annual allowances including carry forward on pension contributions - even though the contributions were made by her employer as part of a redundancy package.  She accordingly had a liability for income tax on this, which the employer (pension scheme) offered to pay on her behalf and which she accepted, and they paid.  However, she should have disclosed this on a tax return and didn't, and the tax was calculated at 40% and some should have been at 45% and she owed them another £6k.

The Inspector dealing with it was charming, sensitive and caring, especially when I responded to his urgent request for a call back from the car park of the oncology ward where I was about to visit my sisters boyfriend shortly before he died of cancer.  "Take your time - respond when its all sorted".  I did, it was sorted, and my client has paid the tax.  HMRC have imposed a 15% penalty for a careless error (the minimum penalty they can impose) and declined to suspend the penalty.  For careless errors only, HMRC can suspend a penalty for 2 years on the basis that the taxpayer undertakes to take steps to avoid careless errors in the future.  I quote from the inspectors letter

"When deciding if a penalty can be suspended it is necessary for you to devise conditions that will ensure you do not make ANY FUTURE MISTAKE on a return.  However, any conditions must be ...SMART ... strictly suspension is not about avoiding the same mistake... consequently it is not possible to suspend the penalty if no realistic future careless inaccuracy can be identified" (my emphases)

This is of course bollocks - and the recent case at the first tier tribunal (June 2016) is "Steady"  which addresses precisely this point.  The Inspectors letter is dated 9 January 2017

They ARE ROBOTS.  Nice people - but ROBOTS.

(1) They have a new system which allows me to read information they hold about a taxpayer.  Particularly state pension, which the doddery old codgers usually get wrong, and HMRC subsequently correct.  This "helpful information" was missing in 50% of cases it should have been there, and comically inaccurate on 90% of the rest.

One of the reasons i packed up being self employed was because they were always getting my Tax wrong. Just aint worth the crap imho.