As others have said, "PEN" = pension. Still trying to work out SH.
I found a few references to "Sacrifice".

SH = ??
PEN = pension
SAC = sacrifice?

http://www.accountingweb.co.uk/anyanswers/question/salary-sa...

http://www.pistonheads.com/gassing/topic.asp?t=1233676

Not sure if this fits your context.

HTH!

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Note added at 44 mins (2013-06-10 15:50:50 GMT)
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Thanks. Some other references which may shed light on the "SH" bit!

May be Stakeholder (SH or S/H)? But I am really grasping at straws here!

Stakeholder pension and salary sacrifice | AccountingWEB
www.accountingweb.co.uk › Any Answers‎
Jul 23, 2003 – Employer contributions are being made into a stakeholder pension by way of ... http://www.accountingweb.co.uk/cgi-bin/item.cgi?id=115993&d=... ... Any pay slip is always done after any salary sacrifice.

[PDF]
Remuneration (Salary/Bonus) Sacrifice - JLT
www.jltgroup.com/content/s2/jlt-eb-practical-guide-to-remun...‎
salary sacrifice instead of making individual contributions to a pension scheme. The employer's .... However, the payslip can be part of the evidence that the remuneration .... sacrifice guidance (http://www.hmrc.gov.uk/manuals/ ... personal and stakeholder pensions, employees pay contributions net of basic rate tax and have ...

Here are some bits of my answer, minus the bit about "shared", which I don't think is right. I think SH is stakeholder, as Taña first suggested.

Pension sacrifice, or "pension salary sacrifice" in full, is explained here:

"Salary sacrifice involves employees' salaries being reduced by the amount of their pension contributions and those contributions instead being paid directly by the employer. The lower level of pay means national insurance contributions are reduced both.
Head of flexible benefits consulting Kim Honess says: “There is a certain critical mass at which implementing pension salary sacrifice is a no-brainer given the savings it delivers."
http://www.moneymarketing.co.uk/news/salary-sacrifice-reaps-...

Salary sacrifice is a kind of legal scam, really. It's a way of reducing NI contributions (and perhaps also tax). There are other kinds of salary sacrifice, but pension salary sacrifice, involving pension contributions seems to be particularly common these days, so much so that "salary sacrifice" alone often means that.

It's becoming so widespread, apparently, that I'm very confident that "PEN SAC" refers to this. I'm much less confident about "shared", because there are hardly any references to "shared pension/salary sacrifice", but I can't find anything else that fits, and "shared" does make sense, in that the benefits are shared between employer and employee: both gain in terms of reduced NI contributions.

"Here's a comment on a forum (evidently British judging by the reference to NI):

"i never normally bother opening my payslips, as i know what they say. but this one i did and i've just noted that the pension bit is actually called "PENSION SACRIFICE". humph, they should put that on the tax and N.I. contributions as well"
http://b3ta.com/questions/offtopic/post976540?highlight=answ...

Stakeholder pensions (a UK phenomenon introduced in 2001):
http://en.wikipedia.org/wiki/Stakeholder_pension_scheme

"Salary sacrifice allows basic rate tax paying employees to increase pension payments by up to 31% at no financial cost. This increase is paid for purely by savings made in Income Tax and National Insurance. In November 2004 the Treasury gave a green light to salary sacrifice that will cost the Chancellor more than £1 billion a year in lost taxes. [...]
Only employees in a money purchase such as group personal or stakeholder pensions or employees with no pension arrangement are considered. However, it is possible for employees with a final salary pension to use Salary Sacrifice."
http://www.sharingpensions.co.uk/corporate_benefits_salary.h...