Template class and inheritance

Hi, maybe someone can help with this one...

I have a templated singleton class from which some manager derives

    template<class S> class Singleton
    {
    private:
        static S * SingletonInstance;
    public:
        static S* Instance()
        {
            if (SingletonInstance == NULL)
            {
                SingletonInstance = new S();
            }

            return SingletonInstance;
        }
    };

Let's say I have one AbstractManager that derives from this singleton,
and a ConcreteManager that derives from AbstractManager. The only way
I found to make this work is having the AbstractManager be a template
class, like this :

    template<class R> class AbstractManager: public Singleton<R>
    {
        friend class Singleton<R>;
                }

and

    class ConcreteManager : public AbstractManager<OpenGLRenderer>
    {
        friend class Singleton<OpenGLRenderer>;
                }

Which is unfortunate because I can't declare non abstract virtual
methods in AbstractManager. I can't also have pointers to the base
AbstractManager class on the ConcreteClass. For example, with code
like this :

Core::AbstractManager<ConcreteManager> * Mgr =
Core::ConcreteManager::Instance();
Mgr->MethodDeclaredInAbstractManager();

The compiler complains with :

unresolved external symbol "public: void __thiscall
AbstractManager<class
ConcreteManager>::MethodDeclaredInAbstractManager()"

Even if the method was declared and implemented in AbstractManager.

So, how should I handle multi-level inheritance from a classe that is
templated?

Thanks! :)
Simon