Hi Tom - Welcome to the forum. There are a growing number of Brits on here and Steve (Smokin Monkey) has given you the link to the UK group within the forum. The UK group is fully integrated into the whole forum and is seen by all members and many of our US colleagues are active contributors. We do, however try to tackle with some of the challenges that are more specific to the UK.
As Al said above, the WSM is ready to so straight from the box and should not need any mods. The only time you may need to mod it is if you are wanting to add an automatic temperature controller in the future. WSMs are great all rounders and will be perfect for your brisket and pulled pork. At competitions they can be seen being used by most of the competitors.
There are several things to bear in mind when smoking with it...
Temperature control is the key. The aim is to slowly bring the unit up to temperature without overshooting. It is easier to raise the temperature than it is to lower it.
To measure the temperature it is best to use a digital thermometer that is recording at the cooking grate rather than relying on the one in the lid. The one that may of us use is the Maverick ET732 or 733
The temperature is finely controlled by balancing the air flow over the coals so that enough heat is generated to get it to temperature and then just compensate for the heat loss through the walls and flue vent.
To help maintain good temperature control over long smokes you should site the WSM is in a sheltered spot out of the wind and also direct sunlight. This isn't always possible but external weather can have quite an effect on the internal temperature - gusty wind will cool it down quite quickly and the difference from the sun shining directly on the unit and it going behind a cloud can be quite marked.
Charcoal choice is also important. For best results you are looking to use a fuel that will urn consistently over a long period of time. A good combination is to use a good quality briquette (e.g. Heat Beads or Weber Premium briquettes) as a base and then use wood chunks or pellets on top for flavour. You can use charcoal or wood as a base however this will burn through significantly faster and will usually result in a temperature dip as you add more part way through the smoke.
1.5 Kg of Heat Beads can give you up to 8 hours of good cooking time
Use the water pan in the WSM. This is not there to keep the inside moist but is used more as a heat stabiliser to smooth out the temperature spikes from the burning fuel before they hit the cooking chamber. You can either fill it up with water however many of us use sand in it instead.
The way you light the fuel is also important. What you are looking to achieve is to only burn sufficient fuel to bring the unit to temperature and then maintain it. There are several ways of achieving this and the most effective is probably the Minion method.
This video gives you a good start on how to set this up - however I find it easier to place something like a baked bean tin in the centre and then remove it to make the well for the lit coals.
Once you have added the lit coals into the centre then put the WSM stack together and leave both the top and bottom vents fully open. From this point monitor the internal temperature regularly. It will require patience as it can take the unit an hour or more to reach cooking temperature, but when it has reached 2/3 temperature it is the time to start regulating so that it does not overshoot. Now close down the top vent to about 1/4 open and half close the bottom vent. The top vent will stay like this for the remainder of the cook and the control will now only be made using the bottom vent ** See below
As the temperature continues to rise towards the desired cooking temperature gradually close down the bottom vent until the temperature has stabilised at the cooking temperature. Do not shut the bottom vent completely though or the fuel will go out. The temperature should now remain stable and should only require minor tweaking every 30-60 minutes.
Depending on the type of fuel used you may need to add more part way through the smoke to give you the required cooking time. Do not wait until the temperature starts to drop before you add more. When it looks as if the fuel is about to start burning down, add a few more briquettes every 15 minutes or so and allow them to catch before adding more. This will help avoid sudden drops in temperature as the new fuel is added.
Hopefully this will give you a good place to start. The more you get compfirtable with managing the temperature the easier you will find it to produce good consistent end results. I am not far from you so if you need any help then just give me a buzz.
** Some people claim that the top vent must be left completely open at all times to avoid "stale smoke" or creosote forming. This is just not true. So long as the top vent is open 1/4 or more you will get plenty of air/smoke flowing through the unit.