I chose to remove all of the existing sealant and replace it with fresh stuff.   It was a pig of a job, heating, scraping and finally cleaning with Mineral Turpentine  I replaced the sealant with Selleys D-Mastic. Do not use any of the silicone based products as they need to be cut away if you ever want to open the lamp assembly in future, and how will you cut the inside layer?  The D-Mastic is nice and soft when fresh making reassembly very easy, if you choose to use the original sealant again you will need to reheat the lamp components and use a fair bit of force to reassemble, but it will save you a lot of time.

Here you can see the cleaned slot in the rear housing and I have removed the low beam lens for cleaning also.

The front glass was a lot easier to clean than the rear slot.  I also removed the parking lamp tube for a quick clean.  

Don't use anything too aggressive for the actual cleaning, and try to make sure you don't have any lint or fingerprints left to annoy you later.

Don't bother too much about cleaning the outside of the glass as you can tidy that later.