I am using Winsor & Newton oil paint and Abteilung 502 odourless Thinner to make my wash, mixing it roughly 90% thinner to 10% paint. Before the wash I coated the model with Vallejo gloss varnish. It goes on fine but when it dries the pigment seems to shrink or clump together and leave gaps. What can I do to stop this from happening?

I'd say to use proprietary thinner with the paint--that is, W&N thinner. I've had good success using cheap oils from Hobby Lobby, along with plain old Turpenoid light, but it's always best to use the same thinners as the paints.
VR Russ

W&N doesn't just have one type of oil, do you have stuff like WINTON or such?

Speaking for myself, I DO NOT LIKE USING OILS in making up my "pin-washes"...

Also, I will necessarily have to agree with Russ; USE the same make of thinners with your paints i.e, TAMIYA thinners with TAMIYA paints, TESTORS thinners with TESTORS paints, VALLEJO thinners with VALLEJO paints, etc. DO NOT mix enamels with lacquers or acrylics. You can get away with mixing certain enamels with oils, but I wouldn't push it. I've also had good luck when I've mixed a TINY bit of weathering powders with my enamels on a separate pallette WHEN I'M BRUSH-PAINTING my figurines- The trick there is to always keep that particular mix moistened with thinners, because adding a bit of weathering powders into your paint mix will dry the paint out very quickly...

Personally, I like to use the TESTORS Model Master II Enamels for EVERYTHING, including my "pin-washes", which I dilute with regular TESTORS Enamel Thinner. For my airbrushing needs, I like to use the TESTORS Airbrush Thinners. Once I have my various mixes in my airbrushes' color cups, I will add a drop or two of WINSOR-NEWTON Matte Varnish into the mix. I don't quite know the chemical reaction involved in my doing this, but the paint seems to flow easier and seems to be more durable after it dries. This may seem backwards, but I like to seal everything with TESTORS Lusterless Flat Spray Lacquer BEFORE I apply any of my "pin-washes" and my weathering powders- It works for me, anyway...

Even with a "flat" coat, weathering powders will tend to "disappear", ESPECIALLY the lighter colors, or they will "frost", or in your case, "clump" when a sealer is sprayed on. The trick is to "mist" your sealer over your weathering powders, so that it "looks like the sealer is dry" before it actually dries... GO LIGHT in your application of ANY kind of a sealer, and you should be able to get better results, overall...

I had this happen when mixing Winton oils with humbrol enamel thinners. The thinner eats away the medium and you are left with pigment floating in thinner, and it tends to pool or clump together.
Try using W&N thinners for your washes, or switch to tamiya enamel washes, or even better vallejo acrylic washes. They do not have this issue and tend to be much easier to deal with (cleanup and control).

FWIW, another option could be the Lifecolor liquid pigments- the set comes with a "Remover" that allows manipulation of the colours even after considerable drying time i.e. I've succesfully used the remover to tidy up after a number of days, not just the 10-20 minutes often suggested or enamels or the almost instantateous toggling between wash and tidy up I've had with the Vallejo washes.

After applying an acrylic satin coat, I touch cheap, artist's grade oils to the corner or line I wish to apply a wash to. Then, I touch the oil with sansodor working my way to the opposite edge of the object. I occasionally have to apply a little more and and work my way around it. I have never encountered any clumping issues.

I use W & N with 502 all the time without any trouble. I wonder if your problem relates to the gloss clear coat . . . too smooth for any "bite"? I do all my washes and other weathering over an acrylic satin coat.