In FT employment, can I set up and contribute to a supplementary SIPP?

Dazed_and_C0nfused

I don't think you're missing anything other than the fact that your pension tax relief is unrelated to your PAYE tax.

If you want to think of it as "cancelling our the tax from PAYE" that's fine.  But it's not what really happens and some people actually get more in pension tax relief than the income tax they have paid in the same tax year.

Secret2ndAccount

You earn £10,000. You pay 20% tax, and take home £8,000
You pay the £8,000 into a SIPP. It gets topped up to £10,000
Some people say you didn't pay any tax. Some people say you paid tax, but got it back. Some people say it's unrelated.
Say whatever you want. £10,000 earned. £10,000 in the SIPP.

Albermarle

Any views on reputable advisers in this field? HL is my platform for ISAs and stocks and shares because I like their set-up, I know they aren't the cheapest around so they are one I've considered. 

Firstly in this area, you have to be careful with nomenclature /wording, or it can get confusing.
Advisers and platforms are two separate things .
Advisers offer regulated personalised financial advice for a fee. They can be an FA, linked to a company and only able to recommend their products, or an IFA , who can choose products from the whole market. Many people prefer to pay for someone else to look after their personal finances, whilst some prefer to do it themselves. For the latter it is better of course to have a reasonable understanding of the issues, although many do not and can make a hash of it. 

An investment platform, is just somewhere to buy, sell and hold investments and they will sort out things like tax relief etc. 

To confuse things a little, some platforms also offer financial advice services, as a separate arm . HL being one of them. 

HL have a good reputation for customer service, their website etc but they are not the cheapest. The charges will have less of an impact to begin with, but as your SIPP grows, you may wish to look at cheaper platforms. Above a certain level it can be better to go with a fixed rate platform ( rather than one that charges a %) .It is easy to transfer a modern SIPP, sometimes there are even cashback offers. Take note though, the choice of investments in the SIPP are more important than a fraction of % saving in charges on the platform. The cost of the investments can also vary a lot.

Albermarle

I also assume I could take 25% of fund size tax free at age 55 if desired?

I am afraid the earliest access age has moved to 57. Otherwise you can then take 25% tax free, although it is often not the best idea to just take it all out at the earliest opportunity. There are various ways to mix and match tax free and taxable money from a pension, and some people use their DC pot primarily as a way of passing on money to heirs. That can all wait for another day though.

This is a good govt website.

Pensions and retirement | Help with pensions and retirement | MoneyHelper